1. Are there available User licenses?
As per the warning, first verify that there are User licenses available. This can be verified from the properties of the license object in Active Directory Users and Computers, or from Active Directory Users and Computers via the 'Defender' menu that is available after selecting the 'Defender' OU and viewing the 'About' tab.
If the assigned count is equal to or higher than the available User licenses, then this warning will be seen in the Portal when trying to assign a new license as there are no available User licenses.
2. Is the license object writable?
If there are User licenses available, confirm that the Defender user license object is writable by the Defender Management Portal Service account. If this is not the case, use the "Active Directory User and Computers | Defender | Delegate Control..." wizard to grant the service account used for the Defender Management Portal the "Token Deployment System" rights.