You have the following dataset.

The correct Level of Detail (LOD) expression to calculate the grand total of sales across all regions would be {SUM(Sales)}. The FIXED expression is not needed here because we are not computing the sum at the level of each region, but rather the total sum across all regions. The FIXED keyword is typically used to aggregate at a certain level regardless of the view.

https://help.tableau.com/current/pro/desktop/en-us/functions_functions_tablecalculation.htm#TOTAL