You get an email after you register on itch. You need to click on the link inside the email to verify the email belongs to you. If you didn’t get this email, try to check the “Spam” folder.
If you still can’t find it, try to request another email in the profile settings.
If you STILL don’t find it, you most likely mistyped your email address, and it went to another address and not yours.
Also small addition but please read error messages. They are often the solution to the problems you are facing.