What version of the eBay app are you using?
If not a recent version (6.9.6.3 or later), try uninstalling, restarting your device, and then installing the latest version from the Google Play Store.
If you are unable to get the latest version, make sure you have a recent version of Google Play Services and Google Play Store:
https://support.google.com/googleplay/answer/9037938?hl=en
If you are still unable to use the eBay app, you should still be able to reach eBay using a browser to go to ebay.co.uk.
