Attempts to assign ArcGIS Pro add-on licenses to ArcGIS Online organization members fail with the following error message:
Error: Could not update add-on licenses for all selected members.
There are expired licenses assigned to members of the organization.
To resolve this issue, unassign all expired licenses from members of the organization. Refer to ArcGIS Online: Configure add-on licenses for one product for steps to do this.
Administrators and organization members with privileges to manage licenses are able to assign add-on licenses to members.