I deleted one of my note folders. How can i recover it?

See this support article:--> https://support.apple.com/en-us/HT209254

If you get no help, restore everything from a recent backup, created before you deleted that notes folder.

No backup = No recovery, sorry.

Regards

Giulio

Delete means just that DELETE. It is gone unless you included it in a recent back-up of the phone.