The two most common issues I can think of:
1.) NetBIOS ports not open which would not allow ePO to connect to the Admin$ share on the target. Windows Firewall turned on? etc...
2.) Credential using to push the agent was fat-fingered or did not have the necessary access rights on that system. Account needs to be a local administrator.
That said, even though it reports that the installed failed, you should be able to dive deeper to find out what error was. Providing that to us will allow us to get you a better answer.