Go get yourself a Yamaha Slient brass rig. That will help a lot.

I take it your playing doesn't give them a trom-boner?

It's a shame all that practice didn't earn you a chance to go out.

What does covid have to do with you playing after 9pm?

"After 9" is a pretty reasonable request. You're living in a complex. You shouldn't be playing loud or resonating music after 9 or 10 anyway if you're not living in a separate abode.

Good thing they were polite enough to message you over the app. Some people would have kicked your door down, and rammed your trombone up your ass. If you're not able to work right now, you can switch your sleeping pattern so you're awake during the day, and practice then. Just tell your neighbors first. Or get one of those rigs other people are suggesting.