Based on the error message it indicates that the access policy does not allow token issuance. This can happen when a user or application tries to access a resource that is protected by a Conditional Access policy, but the policy conditions aren't met. To know for certain what is being blocked, you will need to gather more details.
Under Microsoft Entra ID -> Sign-in logs, you can select the failed sign-in log and view the Conditional Access tab to get more details about why the Conditional Access conditions were not met and which policies applied
Then if you select the policy details you should be able to see which conditional access policy is blocking for users sign in, once you find the policy name go to the conditional access policy and select the policy name and adjust policy accordingly to allow the user to sign in.
If you still do not see enough information to isolate the issue, please let me know.
Also follow the document to Troubleshooting sign-in problems with Conditional Access
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