How do I mark my project as complete?

If it was a project for a client then I'm almost certain the client needs to mark it as complete. You should message them and ask them to do so. At least that's how I did it the last couple of times. They should have that ability on their end.

Hi there,

Please contact us via support@cgtrader.com and we will provide you more information about the situation.

Kindly,
Austėja from CGTrader