Ivan, you can't insert values into an existing column in Tableau. You write a formula that creates a brand new column with the values you want, and you then use that column instead of the original column.
So, something like this:
IF [Ship Mode]='Same Day'
AND [Country]='United States'
THEN 'New Group'
This calculated field works fine for me.
I was then able to build the following view:
See attached workbook.
If you continue to have problems, please attach a packaged workbook so we can troubleshoot.
Calculation.twbx 8.3 MB