This thread is made following a problem encountered by an user which tried to install i-MSCP on Ubuntu Trusty Thar (14.04) with a X server. See Errores al finalizar la instalacion for further details.
It appear that when login as root user with the su command without any option, the default behavior of the Upstart initctl command, including the service command, is to search for user jobs while we would expect that it search for system jobs. This behavior lead to several "Unknown job" errors and of course, i-MSCP installation fail.
To work around this behavior, we can either use the initctl command with the --system option or login as root user with the sudo command, or by using the su command but with the - option that allows to get an environment similar to what we would have if we had been logged directly as root user.
On the i-MSCP side, we'll fix that problem by unsetting the UPSTART_SESSION environment variable but this will not solve cases where the user want restart a service manually. Even worse, due to the behavior described above, any service installed through apt such as Apache2, MySQL ... won't start automatically.
There is plenty of bug reports about this behavior. For instance: