Супровідний лист короткий.

In Ukrainian, the present-tense verb to be is usually omitted in simple sentences like this.

So:

• Супровідний лист короткий.
  literally looks like
• Cover letter short.

but it means:

• The cover letter is short.

This is completely normal in Ukrainian.

Because the two adjectives are doing different jobs.

• супровідний лист is one noun phrase: cover letter / accompanying letter
• короткий is the predicate adjective: it tells us what that letter is like

So the structure is:

• супровідний лист = the subject
• короткий = the description of the subject

In English, this is like the difference between:

• a cover letter
  and
• the cover letter is short

Because they are both masculine singular forms, agreeing with лист.

Here:

• лист is a masculine singular noun
• супровідний agrees with лист
• короткий also agrees with лист

So all three match in gender, number, and in this sentence also case.

Compare:

• супровідна записка коротка = feminine
• супровідні листи короткі = plural

It is in the nominative singular.

That is because лист is the subject of the sentence. The sentence is basically:

• The cover letter is short

In this kind of sentence, the subject is in the nominative. The adjective короткий is also in a nominative form agreeing with the subject.

Because короткий is an adjective, and it agrees with the noun лист.

• короткий = short for a masculine noun
• коротко = usually briefly, an adverb

Compare:

• Супровідний лист короткий. = The cover letter is short.
• Він написав коротко. = He wrote briefly.

So here you need the adjective короткий, not the adverb коротко.

Ukrainian has no articles, so there is no direct equivalent of the or a in this sentence.

That means the exact English translation depends on context:

• the cover letter is short
• a cover letter is short

In real context, one of those will sound more natural, but Ukrainian itself does not mark that distinction with an article.

Ukrainian word order is flexible, but different orders can sound different or even change how the sentence is understood.

• Супровідний лист короткий. = a neutral full sentence: The cover letter is short.
• Короткий супровідний лист = usually just a noun phrase: a short cover letter

So for a basic statement, the original order is the safest and most natural.

In a simple present-tense sentence like this, you normally leave out є.

So the natural beginner model is:

• Супровідний лист короткий.

A version with є is usually unnecessary here and can sound more formal, marked, or less natural in everyday usage. As a basic rule, in present-tense sentences of the type X is Y, Ukrainian often just omits is.