You are registered in the UK, but you are currently signed in to the US site, and are posting on the US discussion boards.
You may want to sign out here and sign back in to ebay.co.uk instead.
If you are still having problems making purchases there, make sure you have a current payment method connected to your account, that any bank cards have not expired, and that you do not need to provide additional documentation.
If you are using the eBay app, try using a browser instead; even if that does not work, it may provide a more explanatory error message.
If you are using a desktop browser, try another to see if you encounter the same problem.
