What is the error?

Yes, only a "Trade or Business" qualifies for Section 179.  The activity of your client definitely rises to that level, so it is allowed, including carryover amounts.

However, on the 4562 ProSeries automatically assumes that rentals DON'T rise to that level.  So you need to manually override the income limitation on the top of Form 4562 to reflect the income/loss of the rentals.  Perhaps that is the issue that is causing your problem???

If you did not change/override the income limitation on last year's 4562, then it is possible that last year's section 179 is wrong (and therefore the carryover amount would be wrong), so you need to check that.