Learn Dutch: We zullen het verzoek uitstellen, aangezien de bijlage ontbreekt. - We will postpone the request, since the attachment is missing.

The word aangezien (since/as) is a subordinating conjunction. This means it introduces a sub-clause, which forces the verbs in that part of the sentence to the very end. That is why the conjugated verb ontbreekt comes after the subject de bijlage.

Can I use omdat instead of aangezien here?

Yes! Grammatically, they work exactly the same way and both push the verb to the end. Aangezien is just slightly more formal, similar to 'since' or 'seeing as' in English, whereas omdat is the standard word for 'because'.

Uitstellen is a separable verb. Why isn't it split into stellen ... uit here?

Separable verbs are only split when they are the conjugated (main) verb in a regular clause. Here, the conjugated verb is zullen (will), which pushes uitstellen to the end of the main clause in its full, unsplit infinitive form.

In English we say 'is missing', but the Dutch sentence just uses ontbreekt. Why is there no word for 'is'?

In Dutch, ontbreken is a standard verb meaning 'to be missing' or 'to lack'. You conjugate it directly, so de bijlage ontbreekt literally means 'the attachment misses'. Trying to translate 'is missing' word-for-word by adding a form of zijn (to be) would be grammatically incorrect in Dutch.