There are a few reasons why you may get a "SSL expired" error message, which are:
1. You're trying to activate the Universal SSL option when signed up through a hosting partner. As of December 9, 2016 all new domains added to Cloudflare via partners have Free SSL enabled. In order to enable Free SSL for existing customers whose domains were added prior to December 9, 2016 it's necessary to delete and re-add the domain to be provisioned automatically.
2. You removed and then added the domain to your account. This will cancel out the earlier request to issue the certificate through our vendor, which can take 24 hours to activate, so that is going to lead to an expired certificate error message. We will automatically attempt to reissue the certs in an expired state, but please take note that this process may also take the standard 24 hours or so for to activate through our vendors.
3. The domain is not pointing to our nameservers. Provisioning of certificates on free Universal SSL requires the domain on our nameservers as a check from our vendor, so the certificate will expire if you move the domain away from Cloudflare.