Over the course of your career, you'll have several people tell you something can't be done. It might be an account exec. Or the client. Or your partner. Or your boss.
If you're told, "It can't be done for this much money, but..." That's fine.
If you're told, "It can't be done by the deadline, but..." That's okay, too. These people are offering solutions.
But if someone's first reaction is simply, "It can't be done," there are three possibilities;
1. That person misunderstands something and I need to explain things better.
2. That person is just lazy.
3. That person should be fired, and find work outside of advertising.
Advertising is no place for people who don't think something can be done. That's what creativity is all about.