Are you trying to edit a Trust K-1 that is in your personal tax return?
You may have to delete the K-1 altogether and re-enter it in the K-1 interview.
See: How do I delete forms in TurboTax Online?
Why does this happen? Once you enter (or import or transfer) certain information, the program creates or updates forms and worksheets that use that information. In many cases, you cannot "change" that information in the interview process - in a manner of speaking, it has been "written in ink" on the forms. You have to delete the form and any associated worksheets in order to correct.