Yes, in Power BI Dataflows, you cannot use DAX. Dataflows use Power Query M for all transformations and calculated fields. Because,

1. Dataflows are designed to handle ETL process using Power Query before the data is sent to Power BI. 2)And DAX is used inside Power BI datasets, reports, and Data marts to create calculated columns or measures after the data has been loaded. DAX is not used in dataflows because it is used to perform calculations in a reporting context when dataset has been loaded, not during data transformation (ETL) which usually happens before loading the data to power bi