Editing the key
In the Score Settings dialog on the Project–Notation Style subpage (Keys category), you can find the “Key Changes for the entire Project” option. If this option is activated, all changes made to the key always affect every staff in the project, i. e. it is impossible to define different keys for different staves (other than the relative display transpose settings for instruments made on the Staff page). Furthermore, on the Staff page, any staff (e. g. a drum staff) can be set to not show key signatures.
Therefore, when you want to edit the key, decide if you want the key change to apply to the entire project, or if you want to use different keys on different staves:
If the key set at the beginning of the track is to be used on all staves, and if any subsequent key changes are also valid for all staves, then leave the “Key Changes for the entire Project” option activated.
If you want to use different keys on different staves, make sure that the “Key Changes for the entire Project” option is deactivated.