Reliant 510 a 50 amp transfer switch tripping first 2 pole 20 amp breaker.

I have a reliant 510 a 10 ckt 50 amp transfer switch. I installed it one by one, no issues. Matched lettering etc.

When I went to turn it on, the generator tripped the gen breaker. Found out it was the 2 pole 20 amp breaker on the transfer switch that was creating the trip.

When I put the power back to utility/ line, everything works fine. Im trying to find out why if any body knows. Ive checked wiring, breakers, etc, i cant seem to find the issue.

If there are any other ways to test this. Older 100 amp panel. No gfci/ afci breakers. Attached to a kitchen outlets for a1 and gd for b2.

Could it be that its on a 2 pole breaker, that may be tripping it? Please help. Thank you in advance

Ps. Im thinking I can replace the pole to single pole 20 amps.