Even though you've mapped Fund codes in Importacular, you could still see this error "Required field missing: Fund."
If that happens, check to be sure that there is not a matching gift that has been created without a Fund.
Go to the Gift drop down and select Match (or you can highlight the gift row and press Ctrl + M).
If the donor's gift has been matched, you will see a row of data for the Match there. Double click to open it. If the gift should in fact be matched, correct the missing Fund code. If it should not be matched, you can delete the Match.