This action cannot be performed because this field is not modifiable

[phanh]

Try to edit the result of a calculation field with a full-access account and get this message "This action cannot be performed because this field is not modifiable"

What I try to accomplish is using calculation to create username and if there are duplicated, append a 1. At the moment, my script is not working so I have to do this manually.

[Fitch]

The result of a calculated field is... calculated; you can't edit it. Perhaps what you're looking for is a text field with an auto-entered calculation?