It may be worth looking at the account type as a possible way forward.

If you have bought in any of your stock with the intention of selling it on eBay, hoping to make a profit, then you are allowed to change the eBay account to a business account.

That may also help with this particular problem, because Royal Mail now appear to have restricted Tracked and Special Delivery services with Age Verification. You do not necessarily need a full Royal Mail business account for that.

I just use Royal Mail Click & Drop linked to my eBay business account, and it gives access to the relevant postage options through that route.

With over 500 items listed, it is worth making sure the account type matches how the account is actually being used.

If they are all just personal possessions being cleared out, fair enough. But if any have been bought in to resell, a business account would be the safer and probably more useful option.