The expression refers to multiple columns. Multiple columns cannot be converted to a scalar value.

A place that expects a single scalar value received an expression spanning more than one column.

Why it happens

• Passing two columns where one scalar is expected.
• Using VALUES(Table) instead of VALUES(Table[Column]).
• A SELECTCOLUMNS/SUMMARIZE result with more than one column flowed into a scalar position.

How to fix it

1. Reference a single column, not the whole table.
2. Use VALUES(Table[Column]) and, if several values may remain, wrap in an aggregator or SELECTEDVALUE.
3. To collapse many values into text, use CONCATENATEX.

Cat = VALUES ( Product )

Cat = SELECTEDVALUE ( Product[Category] )