Learn Dutch: Ik weet niet of er vandaag een gids beschikbaar is. - I don't know if a guide is available today.

This is the main rule for indirect questions! The word of (if or whether) introduces a subordinate clause. In Dutch subordinate clauses, the working verb (here, is) gets pushed to the very end of the sentence.

I thought of meant 'or'. Why does it mean 'if' here?

You are right that of means 'or' when giving choices (like koffie of thee). But when it introduces an indirect yes/no question, of translates to 'if' or 'whether'. Just remember not to confuse it with als, which is used for conditional 'if' statements like 'if it rains'.

Why is the word er used here? The English translation does not say 'there'.

Dutch often uses er to introduce an unspecified subject. Because the sentence talks about 'a guide' (een gids) rather than a specific guide ('the guide'), Dutch inserts er. It is similar to saying 'I do not know if there is a guide available today' in English.

Could I put vandaag near the end of the sentence, like we do in English?

No, that would sound unnatural. In Dutch, time words like vandaag (today) usually appear as early as possible. In this clause, it takes its place right after er and before the subject een gids.