I want to check how many of the entries in A1:A10, are not found to be
existing in another list, suppose B1:B20.

Tried out:

=SUMPRODUCT(--(ISNA(VLOOKUP(A1:A10,B1:B20,1,0))))

but failed. What might be the reason? and what might be its solution except
then a helper column?

--
Best Regards,

Faraz