You get this error because you are trying to divide by zero. Hence, the
second part of your formula is returning a zero. So, to avoid the
error, you need to check if this returns zero and if so you want some
other value returned. This is how:

=IF(VLOOKUP($B$41,'38
MTD'!$B$3:$E$410,5,FALSE)=0,"zero",VLOOKUP($A$41,' 38
MTD'!$A$3:$E$410,5,FALSE)/VLOOKUP($B$41,'38 MTD'!$B$3:$E$410,5,FALSE))

All one formula - replace the "zero" message with one to your liking.

Hope this helps.

Pete