There is a bug right now with the Sort by Column button. If you select a field and it shows as greyed out, but then select a different field and back to the first one, it sometimes becomes available.
Was your issue with the Sort by Column button, or it just didn't fix your graph? If so, you may need to sort your X axis the other way. Right now it looks like it's displaying months in backward chronological order, which should be fixable. In the top right of the visual, can you find an option to sort ascending?
Making the months sort by month number should have fixed the issue. Can you share a .pbix file for this? If you can't due to data sensitivity, can you try recreating this with super basic dummy data and send us that?