https://lmms.io/ is free and open source, and similar to FL Studio (though not as full-featured). LMMS might have a bit of a learning curve, but having used FL Studio in the past I had no problem plonking out some simple tunes (after downloading some free SoundFonts I found on Google).
It should be installable via apt, so it's easy to try it out, anyways!
It should be installable via apt, so it's easy to try it out, anyways!