Hello. A couple of suggestions... are you using an old bookmark to sign in? These expire after a time and need renewed. Might you be using an old version of the app? These too need updated to the latest version eventually. Also worth trying is emptying your browser history and clearing caches regularly. Or completing the action on a different browser, or even using a PC instead of the app. Sometimes it's a case of contacting eBay and renewing your password.
If none of the above is helpful then have eBay call you back free via this link...
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid