I followed the guidelines for importing a CSV file and preparing it on Google Drive. I then inserted the formula for calculating month and got a REF error
Can you make sure column A is formatted as a Date column and also that there isn’t any data lower in column J where you’re seeing the error. Sometimes if there’s stray data in a column where an array is trying to populate information it can’t extend to populate.
If neither of those help can you please send a screenshot of the REF error text?