I assume that this is a Single Member LLC that wants to be taxed as a S-corporation?

In my opinion, reporting it on Schedule C invalidated the retroactive S election.

One of the criteria for being eligible for making a "late" S election is that the business has been treated as a S-corporation.  That means wages and the associated payroll forms, and no Schedule C.  So that means if it was reported on Schedule C (which also likely means no payroll was done), the late S election can't be made because the business was not acting as a S-corporation.