It would appear – as at Summer 2024 – that the great chamber is not available to view internally except upon special ‘guided tour events’ – check out this page for details:
cadw.gov.wales/visit/places-to-visit/barclodiad-y-gawres-chambered-tomb#opening-times
We got very lucky when we were doing the Coast Path, completely by accident we got to Barclodiad when Rhys Mwyn was doing an open day. We managed to get inside and shown around the stones, as well as a good chat with Rhys, who's a lovely guy. It's a shame that access isn't better, but perhaps it's understandable.