"A medical marijuana prescription is nonexistent. Licensed medical cannabis doctors don’t offer them, nor do pharmacies or dispensaries fill them. State governments don’t recognize them, either. The reason the term’s so prevalent now is because it’s associated with medicine."
A letter of recommendation for the use of cannabis is what you can obtain from the doctor if the doctor believes that cannabis would be of benefit to you as a patient.
You can find more information here: https://www.marijuanadoctors.com/blog/marijuana-recommendation-vs-marijuana-prescription/