Make sure you're signing in with an MYOB account or Google account that uses the same email address that's in the invitation email your accountant/advisor sent you. If you don't know which email to use, or want to use a different address, get in touch with your accountant/advisor.
If you've set up two-factor authentication (2FA) for your MYOB account, you'll need to enter an authentication code when signing in. Get help with two-factor authentication.