This error means that cPanel finds yourdomain.com in the server's configuration files and cannot park it on your account.
Please try removing the domain from your account's Addon domains first. If you have another account with us make sure the domain is not configured on it either.
If the above does not help you should submit a support ticket via your SiteGround User Area -> Support -> Open a Support Ticket -> Technical Issues and report your problem so that our support team can assist you.
cPanel is easy to work with when you have the right host to support you. If you need a reliable partner to help you manage your website with cPanel, check out our cPanel hosting services!
