Abban az esetben, ha nincs melléklet, hiánypótlás kell.

Yes — ha already means if, so abban az esetben, ha ... is more formal and heavier than just ha ....

• ha nincs melléklet, ... = if there is no attachment, ...
• abban az esetben, ha nincs melléklet, ... = in that case / in the event that there is no attachment, ...

This kind of wording is very common in official or bureaucratic Hungarian. In everyday speech, people would often just say:

• Ha nincs melléklet, hiánypótlás kell.

So the longer opening is not wrong at all — it just sounds more formal.

Abban az esetben literally means in that case.

It breaks down like this:

• az eset = that case
• esetben = in case
• abban az esetben = in that case

More precisely:

• az is the demonstrative determiner: that
• abban is the same demonstrative, but in the -ban/-ben case: in that

Hungarian often uses this kind of double marking with demonstratives:

• abban a házban = in that house
• azzal az emberrel = with that person
• ebben az esetben = in this case

So abban az esetben is completely normal Hungarian structure.

Because Hungarian uses case endings instead of prepositions in many places.

Here:

• eset = case
• esetben = in the case / in case

The ending -ban/-ben means in.

So:

• abban az esetben = in that case

English uses a separate word (in), but Hungarian usually adds the meaning directly to the noun with a suffix.

Hungarian normally uses nincs as the negative form of van when talking about existence.

• van melléklet = there is an attachment
• nincs melléklet = there is no attachment

Using nem van is generally not correct in standard Hungarian for this meaning.

This is similar to how Hungarian has special negative forms in some common structures. So learners usually just memorize:

• van → nincs
• vannak → nincsenek

Here, nincs melléklet means there is no attachment / no attachment is present.

Because nincs melléklet is an existential statement: there is no attachment.

In Hungarian, when you say that something exists or does not exist, the noun often appears without an article:

• Van megoldás. = There is a solution.
• Nincs idő. = There is no time.
• Nincs melléklet. = There is no attachment.

If you said nincs a melléklet, that would sound wrong in this context. The definite article a/az would suggest a specific, already identified attachment, but existential there is/there isn’t statements normally do not work that way in Hungarian.

Melléklet usually means attachment, enclosure, or appendix, depending on context.

Common uses:

• in emails: attachment
• in official paperwork: enclosure
• in documents/books: appendix or supplement

In this sentence, it most likely means an attached document or required supporting document.

It is a very Hungarian administrative/legal word.

It is built from:

• hiány = lack, missing item, deficiency
• pótlás = replacement, supplementing, making up for

So hiánypótlás means something like:

• supplementation of missing documents/information
• correction of deficiencies
• submission of missing items
• request to complete missing documentation

In official contexts, it often refers to the process where an authority asks you to provide something missing from an application.

So in this sentence, it does not just mean a general abstract replacement. It specifically has a bureaucratic sense.

This is a compact, bureaucratic Hungarian structure.

• kell = is needed / is necessary / must
• hiánypótlás kell literally = supplementation is needed

Hungarian often allows a noun + kell pattern, especially in formal or administrative language:

• engedély kell = a permit is needed
• javítás kell = repair is needed
• hiánypótlás kell = supplementation is needed

So although English often prefers a fuller verbal structure, Hungarian can sound perfectly natural with just the noun plus kell.

A more explicit version could be:

• hiánypótlást kell benyújtani = missing documents must be submitted
• hiánypótlás szükséges = supplementation is required

No, because in this sentence hiánypótlás is not functioning as a direct object.

In hiánypótlás kell, the meaning is closer to:

• supplementation is needed
• a correction is necessary

Here kell works almost like is needed rather than need something.

Compare:

• Hiánypótlás kell. = Supplementation is needed.
• Hiánypótlást kell benyújtani. = One must submit supplementary material.

In the second sentence, hiánypótlást gets -t because it is the direct object of benyújtani.

The word order is natural and standard for a formal statement:

• Abban az esetben, ha nincs melléklet, hiánypótlás kell.

Structure:

1. condition/topic: Abban az esetben, ha nincs melléklet
2. main statement: hiánypótlás kell

Hungarian word order is flexible, but this version presents the condition first and then the consequence, which is very normal.

A shorter version:

• Ha nincs melléklet, hiánypótlás kell.

That sounds natural too.

Because the sentence contains an inserted conditional clause.

• Abban az esetben, ha nincs melléklet, hiánypótlás kell.

The ha nincs melléklet part is the if clause, and it is set off by commas in this formal structure.

If you use the shorter version, you would usually write:

• Ha nincs melléklet, hiánypótlás kell.

So the commas reflect the clause structure, not just pauses in speech.

Yes. The original is very formal and administrative.

Possible alternatives:

• Ha nincs melléklet, hiánypótlás kell.
  A shorter version with the same basic meaning.

• Ha nincs melléklet, hiánypótlás szükséges.
  Very formal: If there is no attachment, supplementation is required.

• Ha hiányzik a melléklet, pótolni kell.
  More verbal: If the attachment is missing, it must be supplied.

• Ha nincs csatolmány, pótolni kell.
  If the context is email, csatolmány may also be used for attachment.

The original sentence is the kind of wording you might see in official instructions, forms, or legal-administrative notices.

Not always. It depends on context.

It can mean:

• in that case
• in that event
• if
• in the event that

In your sentence, because it is followed by ha, the whole expression is best understood as something like:

• in the event that
• if

So although abban az esetben literally looks like in that case, the most natural English translation of the full sentence may not have to copy that literally.