How do I install the Sub Directory from WHMCS Multisite Provisioning ?

I had already activate the WHMCS Multisite Provisioning in my WordPress for customers to make Sub Directory (xxx.com/_____) from our site. When customers order to us, it always show "Is not a valid domain name, alphanumeric and "-" only:", I really don't know what the trouble is.

I would like to automatically create the sub directory when customers finish pay fees to us.

Anyone please give me some suggestion regarding this matter.

Thanks :slight_smile: