Agent installation fails on prerequisites installation with either of the following error:
"Installation of prerequisite CRT 2015 (x64) ENU failed with error code: (1638) Another version of this product is already installed."
"1603: Fatal Error during Installation"
Checking Control Panel, Programs and Features - there's no VC runtime 2015 installed in the list.
The issue may be with Microsoft Redistributable 2017 (x64) overwriting Microsoft Redistributable 2015 (x64).
If you have an older version you can workaround the defect by doing the following:
Contact Quest Support or open a new Service Request online to get assistance if the installation is still failing.