You did not indicate which operating system you are using, but see these two links to determine which version of the eBay app you can use:
https://community.ebay.com/t5/Android-App/Android-app-not-supported-on-older-systems/m-p/30652449#M9...
https://community.ebay.com/t5/iOS-App/iOS-app-upgrade-older-versions-no-longer-supported/td-p/307080...
In some cases you may need to update your operating system or your version of Google Play Services or Google Play Store to be able to obtain the latest version of the eBay app.
For detailed eBay app help, try posting a new thread on one of those two app discussion boards; be sure to include details of your device type, operating system version and current eBay app version. There are actual eBay folks reading and responding on those boards regarding app issues.
If you are unable to get the eBay app to work on your device, you should still be able to go to ebay.com using a browser.
