I'm no expert on the subject, but I would like to complain about the service I've received.

I can't complain, but sometimes I still do.

The people at the complaint desk complain about having to answer complaints.

• The noun of complain is complaint.
• It means:
  • a statement that you are unhappy or not satisfied with something
  • the act of saying or writing that you are unhappy or dissatisfied with something
  • something to be unhappy about : something that people complain about

The customer complained to the manager because their soup was cold.

Complain is a verb.

Melory says that I complain to much.