A function 'X' has been used in a True/False expression that is used as a table filter expression. This is not allowed.

A boolean filter argument to CALCULATE or CALCULATETABLE contains something not allowed there — usually a measure or a multi-column aggregation.

Why it happens

• Putting an aggregation in a boolean predicate: CALCULATE ( [Sales], SUM(Sales[Qty]) > 100 ).
• Using a measure directly inside the boolean filter.
• Comparing across columns inside the predicate.

How to fix it

1. Move complex conditions into FILTER: CALCULATE ( [Sales], FILTER ( VALUES ( Product[Cat] ), [Sales] > 100 ) ).
2. Keep CALCULATE's simple boolean predicates to one column compared against a constant.
3. Remember FILTER iterates a table and can see measures; a bare boolean predicate cannot.

CALCULATE ( [Sales], [Sales] > 100 )

CALCULATE ( [Sales], FILTER ( VALUES ( Product[Category] ), [Sales] > 100 ) )