Such can have issues.
I'd try a new card for a test to see if a new card can be formatted.
I'm not writing this can happen but I've seen many bugs over the years in this area. The camera may have read this card, seen the protect switch was on and is remembering the card. To get around this I would...
1. remove the card, power up the camera and take a few shots to internal memory.
BY THE WAY, I always fill up my camera's internal memory so I force it to use my memory cards.
Now I can turn off the camera, remove the battery and wait for a few minutes. But the battery and memory card back in and see if that cleared it up.
2. Just a repeat but I'd try a new card and format it in the camera.
3. I'd also copy out all the photos from the write protected card then format it in the camera. If the camera still balks then format it on the PC then try to format it in the camera.