Title says it all:
I can't review an app because every time I enter a nickname it states it is already in use. This is my first review and I've never created a nickname before. No matter what nickname I pick it says it's in use. Very frustrating....

There are millions of users out there. You just need to be more creative with your nickname, or settle for adding a number after. Common numbers for popular nicknames are mostly used up too.

The same problem crops up when joining almost any popular site or service. It's probably the reason most sites have switched to your email address as a user name. It's already guaranteed to be unique.

I am not suggesting you use your email address as a review nickname, for obvious reasons.