Learn Russian: Мы должны уточнить, обязательно ли наличие этого документа. - We must clarify whether the availability of this document is mandatory.

The particle ли is placed directly after the word being questioned. Here, we need to find out if the document is mandatory, so ли attaches to обязательно. Notice that it acts like the English "if" or "whether," but it never starts the clause—it always follows the focal word.

There is no verb in the second part of the sentence (обязательно ли наличие...). How does an indirect question work without a verb?

In Russian, the present tense of "to be" (is/are) is normally omitted. The direct question would be Обязательно ли наличие? (Is the presence mandatory?). When turning this into an indirect question, the structure stays exactly the same, just tucked behind the comma.

Could I use спросить (to ask) here instead of уточнить?

You could, but уточнить is more precise in this context. It means "to clarify," "to specify," or "to find out for sure." It is a great verb to use in professional or formal situations when you need to confirm a specific detail.

Why is it этого документа and not этот документ?

It is in the genitive case because it shows possession or relation to the noun before it: наличие (the presence or availability). So, наличие этого документа literally translates to "the presence of this document."