Use this phrase to talk about something bad that a person has done.
Parents, teachers, managers, and other people with authority use "unacceptable" when they're yelling at someone that they're in charge of. Customers also use it to complain about problems they have with a business. For example, if your luggage gets lost by an airline company, you might tell an airline employee:
This is unacceptable!
You can also say that something is "totally unacceptable":
This phrase sounds formal, but it's also a very strong criticism, so only use it when someone has done something really wrong and you're angry with them.