Tableau Nested IF Statement including STRING and INTEGER

I am trying to count distinct count of specific items which have alphanumeric (STRING ID) that matches a certain criteria, i.e uncollected invoices that had been outstanding for more than 45 days

{FIXED [Invoice ID] : (IF([INOVICE CATEGORY] = 1 THEN (IF [Median Invoice age] > 45 THEN 1 ELSE 0 END)}

But I am getting an error