Why when I try to enable a machine a Branch Distribution Point (BDP) is the "Enable as a branch distribution point" checkbox greyed out/ unavailable?

Contributed By: Cliff Hobbs [MVP ConfigMgr/ SMS]
As the BDP functionality is included in the core ConfigMgr Client, when you decide to assign the BDP Site System role to a machine ConfigMgr checks that there is a matching Client record in the Site database:

• If there is a matching record the "Enable as a branch distribution point" checkbox is available.
• If there is NO matching record the "Enable as a branch distribution point" checkbox is greyed out/ unavailable.