Remote push Install SEP client 12.1.5 on iMAC

Hi,

I am trying to do a client(12.1.5) install remote push to iMAC OS X 10.10.3. I got stuck at Login Credentials dialog box as I try to add the computer to the Install Protection Client on: list. I am positive that I use the correct administratior user name and password since I can login to iMAC with it and it is administator account, it just won't accept it.I got the "Login to computer name failed...." message.

Please help