Why does my phone only charge when off

Whatever USB port you’re connecting to is likely not providing sufficient power.

An older USB port won’t offer 5W power, for instance. And which will behave as you’re describing.

Use an Apple 5W or higher-wattage charger, and see if that works better.

Faster charging is available with your iPhone 8, too: Fast charge your iPhone - Apple Support

If the community charger damaged your iPhone circuitry, you’re headed for a repair.

Do try restarting your iPhone: Restart your iPhone - Apple Support

Force restart iPhone - Apple Support ⬅︎ try this...

Okay so did you try turning it off, charging it and while it is charging turn it on.

What are you using to charge your phone?

If it's a USB port on a PC, it might not be providing enough current.

Have you tried a different cable and or charger?

If your iPhone, iPad, or iPod touch won't charge - Apple Support

The above might help...

See this article—>https://motherboard.vice.com/en_us/article/bj34z5/why-counterfeit-lightning-cables-kill-iphones-cheap-iphone-chargers