iPhone says "accessory is not supported" when connected to charger

It's also possible that the charging accessory is not supported by the device - your phone - which Apple details here.

I know in our household some of the older chargers do not work with newer iPhones, for example. This may not apply for your equipment but it's worth checking this out.

The linked Apple support page summarises the possibilities:

"Here's when these alerts might appear:

• Your accessory is defective, damaged, or not Apple-certified.
• The accessory isn't supported by your device.
• Your iOS device has a dirty or damaged connector."

it could

1. dirt or lint in the connector of the iphone, if it is using a match or toothpick to clean it out is easy
2. a defect cable
3. a defect connector in the iphone