What operating system are you using?
What browser is set as your default browser?
https://www.avg.com/en/signal/set-your-default-browser
Sometimes a phone will have an odd browser (like Samsung Internet) set as the "default" browser. The default browser is what the eBay app will try to use when the buyer tries to pay via PayPal. Not all browsers will work with the eBay app. Try Firefox.
