I have attempted to fix it by going into the T4 and setting "Will you claim employment expenses for working at home due to COVID-19 for income on this T4?" to Yes, which seems to be what it was complaining about (as best I can tell) but the error is still there.
I am wondering if it's something to do with the fact that I originally chose the detailed method for submitting Covid-19 expenses but then decided to switch back to the default flat amount method.
