You didn't put in the 2nd Vlookup() to tell the formula that it needs to do the vlookup if the Isna() is false

Also, use 0 instead of "" to avoid the #Value error if item not found

Try:

=IF(ISNA(VLOOKUP($A31,$A$5:$B$27,2,FALSE)),0,VLOOKUP($A31,$A$5:$B$27,2,FALSE))+IF(ISNA(VLOOKUP($A31,$C$5:$D$27,2,FALSE))*0,VLOOKUP($A31,$C$5:$D$27,2,FALSE))