When attempting to add an ESX host to vCenter, you may get one of the following errors:

Error #1
Cannot install the vCenter agent service: Cannot upload agent.

Or:

Error #2
Cannot install the vCenter agent service: unknown installer error

Below is a list of most common errors:

Check whether the host is accessible through port 902/tcp. Check the firewall settings and/or routing between the ESX hosts and the vCenter server.

Check whether there is sufficient space on the ESX disk. If there's not enough space, this often gets logged in /var/log/syslog.log during the installation of a VIB.
Usually the problem occurs with the tmp partition (i.e. scratch disk) being full or almost full. You n  ...