Hello @MichelleL512 ,
When you say edit a user, do you mean a contact, or do you mean an actual user on the account such as an Account Manager or Campaign Creator? If you're having issues with editing a contact, could you please respond to the @ mention email with an example of contacts you're having issues editing?
If you're referring to users like AM and CC, it looks like you're working in a 3rd party hosted account, which may have certain restrictions on account management aspects (varies from 3rd party to 3rd party). If you continue to have issues, I'd suggest calling our general support line so they can screenshare and troubleshoot with you live when the issue is happening.
You may also be running into connectivity issues, since I was just able to add and edit additional users in my own account without any issues.