On form 1120, I have got a difference between Line 28 on form, and Line 10 on Schedule M1. My tax due is the same on both forms

See https://www.irs.gov/instructions/i1120#en_US_2021_publink11455td0e5664

What (if any) figures are on Lines 6-9?

are lines 1 and 10 on m-1 the same. if so there's a missing entry on any of the M-1 lines 2-5 or 8 and 8

if you are using the cash basis for reporting income line2 on the M-1 should reflect the federal taxes actually paid in 2021, not your tax liability.