If you are using iOS 15, see instructions here for installing a compatible version of the eBay app:
https://community.ebay.com/t5/Mobile-Apps/Can-t-log-on-ebay/m-p/34718852/highlight/true#M49153
If your operating system does not support the latest version of the eBay app and you cannot update your operating system or use a previous version that is still supported, it should still be possible to use a mobile browser to go to ebay.com or ebay.co.uk and use eBay that way.
If eBay browser links are opening in the app automatically, iOS users should be able to tap and hold an eBay browser link and select "don't open in the app" to prevent that from happening. Android users need to go to the device Settings: Apps: eBay: Open by default: Open supported links and select "don't open in the app".
