This error message indicates that the "include" mechanism was used in the SPF record, but the included domain either has no SPF record or its SPF record failed to be resolved. In this case, the receiving mail server may consider the SPF check as failed because it cannot determine whether emails from this domain are allowed.

Therefore, it is recommended to check if the included domain has a correct SPF record and ensure it is properly configured. If the domain is no longer in use, it can be removed from the SPF record.