After applying the Microsoft updates from June 13th 2017, Task Server Service cannot be installed on potential Site Servers.
No errors, but the option to select the Task Server service will be grayed out.
The June 13th Updates from Microsoft include an upgrade to .NET Framework 4.7.
(From that article):
This version of .NET Framework is not tested with Task Server and is not recognized by the Potential Task Server filter.
To verify if this is the cause refer below:
Downgrading to a supported version of .NET Framework will be required.
Supported versions are 4.5.x and 4.6.x. Refer to page# 14 in IT Management Suite (ITMS) Support Matrix HOWTO9965
To uninstall the upgrade, the respective KB numbers listed above needs to be uninstalled from the Windows Updates on the potential Task Servers. Then verify that the version in the registry lists 4.5.x or 4.6.x, if not download and install one of those version's from Microsoft.