Lacerte seems to be computing Line 3 in both Part I and Part II correctly.  I can't see how Part II, Line 3 can be negative based on the Code and mathematically, how the form is set up.

If Net CFC Tested Income is negative, it doesn't flow to Part II.  NDTIR also cannot be negative; when Specified Interest Expense is larger than DTIR, Line 2c would just be zero.  The end result is that Line 3 of Part II would be zero and nothing will flow to Line 21.

Can you post the amounts you have on the form?