I downloaded and ran the 64-bit installer (Octopus.2019.7.0-x64.msi) on a Windows Server 2012 R2 box and after the installer had finished, the Octopus Manager refused to start, saying “Windows PowerShell 2.0 or above does not appear to be installed”.
I can confirm that both PowerShell 4.0 and 2.0 engines are listed in Windows Features as installed. PowerShell version is actually 5.1 and PowerShell is on the System path ("%SYSTEMROOT%\System32\WindowsPowerShell\v1.0").
So after some investigation, we were able to reproduce this issue.
To fix this issue you can either, install NetFramework 4.7.2 or install KB4041085/KB4049017 OS patches.
We are planning to put out another release that will update the minimum requirements to NetFramework 4.7.2, so maybe the preferred option right now is to install NetFramework 4.7.2 and I would also strongly recommend keeping all OS patches up to date.
Once again, we are very sorry for the inconvenience.