You haven't used your account for selling for a long time, and sometimes, if not used for 60 days, it can deactivate the account.
You need to contact Customer Services, and see if they need any extra verification from you to prove you are the genuine owner returning to sell, as sudden selling activity on a dormant account, can be a sign that the account is being hijacked.
Contact CS here:
https://www.ebay.co.uk/help/eua?id=5190
@michele220875