If you are trying to deploy the SCCM agent I found one PC that that just would not install the SCCM Agent but instead would error out with: ERROR: Some of the selected resources are not assigned to any site when the PC plainly was assigned to a site. In case you care, I was using both SCCM 2012 and the newer SCCM 1511.
Fortunately the solution was quite simple. On the SPECIFY CLIENT PUSH OPTIONS page of the INSTALL CONFIGURATION MANAGER CLIENT WIZARD, select the INSTALL THE CLIENT SOFTWARE FROM A SPECIFIED SITE option and then select a site. Bingo.
I found several sites were of assistance in cracking this annoying problem, but THIS one was the most helpful and clear.