I keep getting a pop up ‘this accessory may not be supported’. This just started today

see if this support article helps

If you see an ‘Accessory may not be supported’ alert - Apple Support

What kind of accessory? If this is something that plugs into the lightning port on the iPhone, take a bright light and check to see if there is any dirt, lint, or other debris in the port area. You can use a non-conductive instrument, such as a wooden toothpick and see if you can remove any debris.

Okay, so it is the charging cord. Did you follow my instructions to look for debris in the charging port. That will trigger that response.