There could be multiple reasons for this,
1. User does not exist and new user creation is disabled : Go to User Groups tab, uncheck Disable User Creation.
2. Plugin License expired: Update plugin license and try login again.
3. Wrong Domain Mapped : Go to Sign In Settings tab, check what is configured against Allow Domains, if the user domain is missing in the settings, please update it. You can add multiple domains separated by semicolon (;).
4. Group permissions are not assigned to logged in user: Check User Group permissions under User Management. Assign one of the default group to user under User Groups tab.
5. Email address not found in the Provider response: Check Test Configurations under Configure OAuth tab, if email not found in the response then verify your Provider settings and send email attribute in the OAuth/OIDC response.