microsoft access and mysql are two entire different database systems.
you may still be able to use COALESCE though. COALESCE says to check for a value in the column and if the value is NULL (note that isn't an empty string like ' ', or '' or a space) then to use something else indicated.
this is how I'd write it in mysql, you can probably adapt it to access:
so essentially if there is a null in the enddate field then the current_date is calculated instead.