Create a local copy of your Data source, then you can EDIT. You may also try to Create Duplicate Of Your Field and Edit.
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This is likely because you're using a published Tableau Server data source. They cannot be edited directly. If you need to edit the data source, then you'll need to create a local copy of the data source, make your changes, then publish it. Follow the steps in this article: Editing a Published Data Source | Tableau Software