Domain mapping a domain on the same server

I recently (a week or more ago) bought a new domain, with the same hosting company, pointing to the same server of the WP Multisite. When I come to map the domain, I get the error message saying ''Domain name is unavailable to access''. The A record is pointing to the right IP though. Am I missing anything? Is there a different procedure for domains on the same server? Not sure where to go or what to do from here. Please help as this is urgent.

Regards,

Paul