Learn Portuguese (Portugal): Não sei onde posso fazer a reclamação. - I do not know where I can make the complaint.

In Portuguese, it doesn't! Unlike English, which changes the word order ('Where can I...' vs '...where I can...'), Portuguese uses the exact same word order for the direct question (Onde posso fazer a reclamação?) and the indirect question (Não sei onde posso fazer a reclamação.).

I thought phrases expressing doubt like Não sei triggered the subjunctive. Why is posso in the normal indicative here?

While expressions of doubt often trigger the subjunctive, indirect questions starting with question words like onde, quem, or como usually take the indicative. You are stating a factual limit to your knowledge (the fact that you do not know the location), rather than expressing doubt about whether an action will happen.

Could I use the word queixa instead of reclamação here?

You could, but they have slightly different everyday uses. A reclamação is a typical consumer complaint, like something you would write in a shop's complaints book (livro de reclamações). A queixa is usually more serious or formal, such as filing a police report (apresentar queixa).

Why do we use the definite article a in fazer a reclamação instead of uma?