You need to make sure that your domain name/sub-domain is pointing to your web hosting account/server hosted with us. You can not issue/validate certificates for domains that are not pointing to us!
The main issuing limit is Certificates per Registered Domain, (20 per week). A registered domain is, generally speaking, the part of the domain you purchased from your domain name registrar (us). For instance, in the name www.example.com, the registered domain is example.com. In new.blog.example.co.uk, the registered domain is example.co.uk.
If you have a lot of sub-domains, you may want to combine them into a single certificate, up to a limit of 100 Names per Certificate. Combined with the above limit, that means you can issue certificates containing up to 2,000 unique sub-domains per week !
Let’s encrypt, also have a Duplicate Certificate limit of 5 certificates per week. A certificate is considered a duplicate of an earlier certificate if they contain the exact same set of hostnames, ignoring capitalization and ordering of hostnames. For instance, if you requested a certificate for the names [www.example.com, example.com], you could request four more certificates for [www.example.com, example.com] during the week. If you changed the set of names by adding [blog.example.com], you would be able to request additional certificates.
There is a Failed Validation limit of 5 failures per account, per hostname, per hour.
You can create a maximum of 10 Accounts per IP Address per 3 hours. You can create a maximum of 500 Accounts per IP Range within an IPv6 /48 per 3 hours.
You can have a maximum of 300 Pending Authorizations on your account.
If you’ve hit a rate limit, we don’t have a way to temporarily reset it. You’ll need to wait until the rate limit expires after a week. We use a sliding window, so if you issued 10 certificates on Monday and 10 more certificates on Friday, you’ll be able to issue again starting Monday. You can get a list of certificates issued for your registered domain by searching on http://crt.sh, which uses the public Certificate Transparency logs.
For more information you can check the official documentation about this at: