If you are unable to install the SCCM Client manually, it may be because the client machine is blocking the install with the Windows Firewall. To install the SCCM Client, either turn off the Windows Firewall momentarily or allow port 443 from the SCCM server to the client.
Also, if you have installed the SCCM Client, but it is listed in a collection as having no client installed, it may be that you have a duplicate instance of the SCCM Client in the SCCM database. Search the All Systems collection in SCCM for the client computer and delete the duplicate listing as having no client installed. You should then be able to re-add it to the collection you need.