I see on the Schedule M-1 input screen you can use -1 to override the M-1 automatically generated. It seems like the M-1 for income on schedule K not recorded on books is only needed if you have a situation where the credit claimed on a 2020 941 will be refunded in 2021. Then for 2021 you would need to do a reversing M-1 when the income is actually received. If all credits were received in 2020 then the M-1 doesn't seem necessary and can be overridden.