I had to ensure Barclays had a fully filled in mandate from me, including my signature. After this, the open banking feed worked. (As posted above, ensure you’ve used the PINsentry or Mobile PINsentry to log on to your Barclays online banking)
I use a Mac, and don’t have Java installed. Try using Chrome as your browser to connect to open banking, as it worked fine for me - it’s possible Barclays aren’t fully supporting Safari on a Mac as the browser.
Also, Barclays support don’t generally understand open banking at all, hence the fall back to “not supported”, but if you insist they escalate it to your business manager, it can get sorted out.