Learn Spanish (Latin America): No estoy para soportar otra queja. - I am not in the mood to put up with another complaint.

The structure estar para + infinitive or noun expresses readiness, capacity, or being in the right frame of mind for something. When used in the negative (no estar para), it translates perfectly to "not being in the mood" or "not having the patience" for whatever comes next.

I always mix up estar para and estar por. Could I use estar por in this sentence?

Not without totally changing the meaning. Estar por + infinitive generally means you are about to do something or inclined/in favor of doing it. Estar para, as seen here, focuses on your capacity or mood. For example, estoy por quejarme means "I'm about to complain," but no estoy para quejas means "I'm not in the mood for complaints."

Doesn't soportar mean "to support"?

It is a very common false friend! While it can mean to physically support weight (like a pillar holding up a roof), in everyday conversation soportar means "to put up with", "to bear", or "to stand". If you want to talk about supporting a person emotionally or backing an idea, use apoyar.

Why is it just otra queja instead of una otra queja for "another complaint"?