Code:
=IF(COUNTA(G7:G64)="","-",(COUNTIF(G7:G64,"A")*VLOOKUP("A",Cover!G68:H71,2, FALSE)+COUNTIF(G7:G64,"B")*VLOOKUP("B",Cover!G68:H71,2, FALSE)+COUNTIF(G7:G64,"F")*VLOOKUP("F",Cover!G68:H71,2, FALSE))/COUNTA(G7:G64))
Change the 0 to "" in your formula, after the =, then set the number format to accounting, symbol, none.