Hi Jelena,
thanks for your feedback.
I could not reporduce your issue and was able to delete a user as expected. To check your specific case, please provide more details by E-Mail: Which user you want to delete by which user.
Users with important functions can not be deleted, see also the help for the application (which can be accessed by the question mark in the upper right corner of the app):
https://support.sap.com/users-authorizations/help.html#userdel
However, we are open to suggestions how to make the text in the popup more understandable.
Kind regards, Arno