If you get the below error, then this will most likely be because there are not enough third party licenses on Equitrac license assignment within system manager.
It may be that also, if there was one in use previously and this has been replaced, then you will need to remove that assignment from the list, then refresh the new assignment should appear.
Once you have a new third party license installed or free up once that is no longer in use, it will allow the user to login again.
Below is what the assignment view under licensing will look like if this is the case. It states one is not assigned.