Unable to format USB disk (Error: -69877: Couldn't open device)

I have an external USB stick (Kingston DataTraveler 3.0). I can read the files on it (I have some movies that I can play) but I can't add anything to it or delete anything that's on it. I've tried to format it using Disk Utility but I keep getting this error:

Error: -69877: Couldn't open device

Nothing I found online seems to work. It is very annoying because the disk is obviously fine since I can access all the files. Does anyone know anything I could try? Thanks so much.

Solution 1:

Try formatting in another computer just in case, but the chances are a write error has triggered the firmware write protect, in an attempt to safeguard your existing data.
This, to all intents & purposes, irreversible.