Could you give us more information? Why do you have more than one value per month? If each value belongs to a different entity then you have more than one series in your data and you would have to reshape it into a wide format.
If you need more specific help, please provide a minimal REPRoducible EXample (reprex). A reprex makes it much easier for others to understand your issue and figure out how to help.
If you've never heard of a reprex before, you might want to start by reading this FAQ: