...check your derailleur limit screws, particularly the Low screw on the rear derailleur (Park Tool's how-to). Because reasons.
This was an expensive oversight, I think the final bill is around $300 (including a tune-up to fix a bunch of other assembly/tuning issues).
I also suggest removing the bottom bracket and liberally greasing the threads. No big surprise to find they're bone-dry raw aluminum (after all, grease is really expensive in China )