Incorrect parameter count in the call to native function ‘ISNULL’

The MySQL equivalent of ISNULL is IFNULL

If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns
expr2.

Maybe also look at SQL NULL Functions

The ISNULL from MySQL is used to check if a value is null

If expr is NULL, ISNULL() returns 1, otherwise it returns 0.