Do not delete users
Users should be made inactive not deleted. JIRA uses a pointer to the user’s DB entry to display user information. If you delete a user when you open a JIRA issue the user worked on anywhere the user that would be displayed will cause a SQL error. Even if the user never logged on or were assigned a ticket the history of the ticket will get an error when you display it.
Just to expand on this, deactivation is the way to go or just remove the product access from the user so it doesn't count towards a license. Product access refers groups that are given application access to Jira. You can do this from the "Admin hub".
There are many issues that may arise as a result of a delete hence the caution.However deleting a user on cloud right now doesn't remove the user's property entirely and it simply just turns it inactive.
Please can you remove the image above as it contains email address of your users. The reason why you get that error is because for you to remove an Org admin, you will need to first remove the Org admin ability from your settings as show on the below image.
After that, go to your user hub https://<your-instance>.atlassian.net/admin find the user and remove them from the site-admin group.
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