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I see that you are using Jira Service Desk Cloud Free and checking the details of your site, there are already 3 licensed users, that’s why you are receiving this error.
You can find more details about the free plan on the link below:
With that said, to add new users it’s necessary to uncheck the “Jira Service Desk”, so they won’t receive a license and the user will be created.
If you need the new user to have a license, you need to remove the group jira-servicedesk-users from another user, so one license will be available and you will be able to add a newly licensed user.
If you have any other questions regarding this matter, please let us know.
The error is very generic and there can be many root causes.
I suggest checking the application logs ( JIRA-HOME/log/atlassian-jira.log ) and see if there are any errors at the timestamp when you reproduced the issue.
In addition to that, here are other discussions on the community with the same error which might help :
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