Had same issue. Resolved it. Set the value in the T4A Box 20 to $0.00, and enter the value that was supposed to be there on a business/professional/commission income. It appears that any non-0 value entered into T4A Box 20 will make UFile complain.
Suggestions:
1) Reword the hint on the T4A to "report this amount as a T2125 - Business of Professional income INSTEAD"
2) Disable the entry box on the T4A, since any number there will cause issues