mibunsyoumeisyo to inkan wo wasuretara, madoguti de tetuduki dekinaikoto ga arimasu.

Because 身分証明書 と 印鑑 is treated as one combined object: ID and seal.

• と joins the two nouns.
• を marks the whole noun phrase as the thing affected by 忘れたら.

So this works like:

• 身分証明書と印鑑を忘れたら = if you forget your ID and seal

You do not need を after each noun.

Yes. Here と is the normal particle for linking nouns in a complete list.

So:

• 身分証明書 と 印鑑 = ID and seal

This sounds like both items are being named specifically.
If the sentence used や, it would sound more like things like ID and seal, which is less exact.

忘れたら is the -たら conditional.

It is built from the past form:

• 忘れる = to forget
• 忘れた = forgot
• 忘れたら = if/when someone forgets

Even though it uses the past form, it does not mean the whole sentence is talking about the past. In this pattern, -たら means if or when.

So:

• 忘れたら、できない = If you forget them, you can’t do it

This is a very common conditional form in Japanese.

たら is very natural for a practical condition like this: if you forget X, then Y may happen.

Compare the feel:

• 忘れたら = if you forget
• 忘れると = when/whenever you forget, this happens
• 忘れれば = if you forget

In signs, instructions, or everyday explanations, たら often sounds very natural and conversational. It fits well when one situation leads to another.

窓口 means a service counter, reception desk, or office window where procedures are handled.

The particle で marks the place where an action happens.

So:

• 窓口で手続きする = to do the procedure at the counter
• 窓口で手続きできない = to be unable to do the procedure at the counter

Here, で is not to or at in the sense of destination. It marks the location of the activity.

手続き is a verbal noun, so it can behave a bit like a verb-related expression.

That means these are common:

• 手続きできない
• 手続きができない

Both mean cannot do/complete the procedure.

Using を with できる is much less common in standard Japanese. With できる, Japanese often uses:

• no particle in casual compact phrasing, or
• が to mark what is possible/impossible

So 手続きできない is very natural here.

Because the meaning is cannot do, not do not do.

• 手続きしない = do not do the procedure
• 手続きできない = cannot do the procedure

This sentence is about lack of ability or permission due to missing required items, not about choosing not to do the procedure.

Literally, it means there are cases when it is not possible.

So the nuance is:

• you may not be able to do the procedure
• in some cases, the procedure cannot be done

This is softer and more realistic than saying it is always impossible. It suggests that it depends on the office, the procedure, or the circumstances.

It is a common way to give warnings or official guidance without making an absolute statement.

Because ことがあります is a set pattern.

The structure is:

• [clause] + ことがあります

Here the clause is:

• 手続きできない = cannot do the procedure

Then that whole clause becomes a noun-like unit with こと:

• 手続きできないこと = the fact/case of not being able to do the procedure

And then:

• 手続きできないことがあります = there are cases where you cannot do the procedure

So あります is not canceling the negative. It is saying such cases exist.

Japanese often leaves the subject unstated when it is obvious from context.

Here, the implied meaning is usually something like:

• you may not be able to complete the procedure at the counter, or
• the staff may not be able to process it for you

English usually forces you to choose a subject, but Japanese often does not.

That omission is completely natural here.

Not exactly.

印鑑 is a personal seal or stamp used in Japan instead of, or alongside, a handwritten signature.

Depending on the situation:

• some places accept a signature,
• some require an 印鑑,
• some no longer require one at all.

So it is an important cultural word, and it often appears in official or administrative contexts like this sentence.

Yes, できない場合があります would also be natural.

The difference is mostly nuance:

• ことがあります = there are times/cases when this happens
• 場合があります = there are cases/situations in which this happens

場合があります often sounds a bit more formal or more like official written notice language.
ことがあります is also common and natural, and it gives a general may happen feeling.

In normal Japanese writing, there are usually no spaces between words.

So the standard writing would be:

身分証明書と印鑑を忘れたら、窓口で手続きできないことがあります。

The spaces here are just for learners, to make the sentence structure easier to see.