The "Address does not exist" error typically indicates that your Hide My Email alias has been either deactivated or permanently deleted. To attempt a fix, first navigate to Settings > [Your Name] > iCloud > Hide My Email and scroll to the very bottom to check the Inactive Addresses list; if you find the email there, simply tap Reactivate Address to restore it immediately.
If the address was created automatically for an app login, you should also check Settings > [Your Name] > Sign in with Apple to ensure forwarding is still enabled for that specific service. If the address is missing from these lists, it has likely been permanently deleted and cannot be recovered; in this case, you can contact Apple Support to confirm its status, but your most effective solution will be to contact the customer support team of the website you are trying to access and verify your identity to update your file manually.