Welcome to the Community,
some more details would help. Which vCenter Server Version do you use? Which user account do you try to login with?
When you first login after installation, you will have to login using the user account that was created during the installation, e.g. email@example.com
It looks like you that username you are trying to login to vcenter is not having necessary permissions. You first need to check if:
1.) you are able to access your vcenter server with firstname.lastname@example.org account ?
2) If yes, then you need to check Your user permission in vcenter Group settings as it is allowed or not.
please do let us know if you still have issue.