If you can't change anything related to timecodes, it means the "Lock timecodes" is turned on in your project settings.
Open your project's settings:

Go to the "Cues" tab, uncheck "Lock timecodes" and click "APPLY":

If you can't change anything related to timecodes, it means the "Lock timecodes" is turned on in your project settings.
Open your project's settings:
Go to the "Cues" tab, uncheck "Lock timecodes" and click "APPLY":