To create a boolean calculated field to place on the data source filter, you should use the formula USERNAME() = [Tableau_user]. This formula will return TRUE if the current Tableau user name matches the value in the Tableau_user field, and FALSE otherwise. You can use this formula as a data source filter by dragging it to the Filters shelf and selecting TRUE from the menu. This will filter the data to show only the rows that are relevant to the specific Tableau user.
The other options are not correct for this scenario. NAME([Tableau_user]) is not a valid function in Tableau. ISFULLNAME([Tableau_user]) = USERNAME() is not a valid expression in Tableau. 'S' + STR([Sales]/1000) is not a boolean expression, but a string expression that converts sales to thousands with a prefix of "S". Reference: https://help.tableau.com/current/pro/desktop/en-us/filtering_datasource.htm https://help.tableau.com/current/pro/desktop/en-us/functions_functions_logical.htm#USERNAME https://help.tableau.com/current/pro/desktop/en-us/calculations_calculatedfields.htm