That error usually means that you entered data manually into a cell that is being filled with an ARRAYFORMULA(). I’d delete whatever is in cell F20 and see if it starts working again for you.
I had the same issue in my Category Tracker page with cell B21, under “Type / Group” for no apparent reason. It looked like the cell was empty, but after reading this solution, I selected the cell and hit delete anyway, and it fixed it. Maybe there was an accidental space typed in it? ¯_(ツ)_/¯
If you mess up a sheet badly enough, rename it for backup and then import it again from the Tiller Money Feeds extension. A lot of these sheets run on automated data fills and are not designed for manual entries. Stick to filling in the green background cells and Transactions sheet until you get the hang of it.