when i try to generate the certificate it give me theb following error and when i checked the reason it give me ( Name Error - The domain name does not exist) what it mean by domain name doesnt exist
That sounds like a DNS error.
Does the domain exist in global DNS?
Error 403 usually means an authentication error.
But it is hard to say whether it was due to your site requiring authentication (valid login) or the LE authentication request just failed (for some other reason).
You don't really show enough useful information for anyone to help you with.