You're very close: you just need to repeat the VLOOKUP for the "TRUE"
argument of the IF. The first VLOOKUP is used with the ISERROR as the
logical test. The formula should read "If there is no error, then run
the VLOOKUP and return the result". Try modifying your formula to
=IF(ISERROR(VLOOKUP(F12,CSSP!A10:F2008,4,FALSE))," 0.00",VLOOKUP(F12,CSSP!A10:F2008,4,FALSE))