I need to update my email address but prompts an error:
I searched if the email is already been taken but no results found, neither in requesters nor agents list
This problem occurs if the User profile was trashed. This won't reflect in search result as the search excludes deleted Tickets/Users.
You can find the User profile from Admin>Contacts and filter for Deleted Contacts, once you restore it, you should find the option to update the profile.
Hope this helps!!