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You can hardcode the month number,names into a text input tool and then use Alteryx's find and replace tool to either replace your numerical month field or to just add a new column with the month name without getting rid of the numerical field. If you want to store the month number as a string, make sure that after creating the text input tool you use a select node to convert the month number into a string there as well.
Instead of replacing the field in the Formula tool, create a new field. I don't have your excel file so I can't test it out, but I would guess it's because you're outputting a string to a numeric field.