If you have received an admin invite and receive the above error the following should be checked.

Logout from all Microsoft accounts

Try accepting the invite in a private Browsing Session

Use Chrome or Edge

You may also find that you have had admin access granted for a different client/tenant account. 

We have also noted that some email clients may modify the invite link. You can right-click and copy the full link and paste this direct into a private browsing session.

If none of the above help, please raise a ticket via the Help Widget.