One way to 'know' you won't regenerate your existing tracks is to go to 'Track- Make all BiaB tracks regular tracks'. It's a right-click option.

Once done, you can select any track(s) or section of a track(s) and generate to your hearts content.
The complication/confusion comes when RB tries to act like BiaB & generate all the blue BB tracks for whole song as a default behavior.

Once I am in RB I want to leave this behavior behind, so I use the 'Track-Make all BB tracks regular tracks' option. Then you are working in more of a DAW environment.
You can still generate whatever you want in whatever style you want, but you don't risk accidentally regenerating tracks you wanted to keep. You have to intentionally select what you want to change.

The warning you mention is likely because one of two things happened.
Your selected Style doesn't have the desired content (so pick a different Style that has the MIDI bass or whatever)
Or the Track Type is wrong; you are trying to generate MIDI on an audio track or vice versa.

Make sure you select Generate MIDI if you want MIDI, and make sure the track is a MIDI track.
If it says there is no MIDI bass track so nothing was generated, select a different style that has it.
Every style is different.

Edited by rharv (09/10/20 03:57 PM)
Make your sound your own!