I am assuming that you are doing a custom install. Correct? If yes, i would try the below steps:
1.Try to resolve the FQDN (Fully Qualified Domain Name) of the SSO Server that you installed earlier from the Server on which you are installing the Inventory Service. I would also see that the clocks (SSO,AD and Inventory Server) are synched
2.If the above is successfull, move to the next step, else register the SSO Server name and IP in DNS and try installing Inventory Service again.
3.I would try to restart the SSO Service and try installing Inventory Service again.
4.Ensure during Inventory Service Installation,On the vCenter Single Sign On Information page, you are providing the SSO Administrator user name (default is admin@System-Domain) and password. Ensure that the Lookup Service URL is correctly specified, this will take the form of https://<SSOServer FQDN or IP>:7444/lookupservice/sdk
Hope this helps
Thx