Sometimes, when working with DAX, you might get the following error:
The expression refers to multiple columns. Multiple columns cannot be converted to a scalar value.
This error occurs whenever the DAX engine was expecting a single value, or scalar, and instead received a table of values instead. This is an easy error to make because many DAX functions, such as FILTER, SUMMARIZE and ALL, return table values.
Eugene lays out when each scenario occurs, so check it out.