Initially the reply from FMP was that you could only report an error by using the link attached to the transcript.

I think they must have received a load of complaints about errors found by people who purchased the images, so they have now set up a system for those errors to be reported as well.
https://www.
findmypast.co.uk/help/articles/4415870561041-how-was-the-1921-census-transcribed-