Steering angle sensor needs to be reset because of your new rack. If by doing the rack they've disconnected the airbag then that may need resetting as well, doesn't matter on some vehicles even of you've disconnected the battery.
Abs likely to be a wheel speed sensor or pump issue
The horn could be a connection not put back, wouldn't be the first mech to forget connecting it. In any case check the fuse box for blown fuses before opening the horn up.
If forscan can't rest them then it may need dealership tools or equivalent.
Hope it helps