This issue is not resolved, it still happens on Unity Hub 3.9.1
This is beyond embarrassing.

For the time being, here's a workaround (which also proves my analysis of this bug)

https://forum.unity.com/threads/workaround-install-failed-validation-failed-hub-3-8-x-and-earlier.1589106/

I can confirm, your bug is as follows:

Unity HUB tries to run powershell 1.0 (using its argument format) usually installed here:

PS P:\> C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe Start-Process

But it tries to run this using just powershell.exe (or pwsh.exe) from the path, which is version 7 on many reasonably well-maintained developer machines.

Please note that this bug, in this form, has existed and kept me from installing editors through Unity Hub on Windows 11 Pro since at least Hub 3.6.x or 3.5.x, in the middle of last year.

The new Hub version only added some logging that made it much clearer what was going on, and also how TRIVIAL the nature of this bug is.

On my machine, it seems that powershell fails (using the new Unity Hub 3.8.0 (7c88724)) being invoked; either via pwsh or powershell (.exe).

The error is that it is being called as follows:

pwsh $COMMAND

... instead of ...

pwsh -Command $COMMAND

... which results in this error message:

PS P:\> pwsh Start-Process $something
The argument 'Start-Process' is not recognized as the name of a script file. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.

Usage: pwsh[.exe] [-Login] [[-File] <filePath> [args]]
[-Command { - | <script-block> [-args <arg-array>]
| <string> [<CommandParameters>] } ]
[-CommandWithArgs <string> [<CommandParameters>]
[-ConfigurationName <string>] [-ConfigurationFile <filePath>]
[-CustomPipeName <string>] [-EncodedCommand <Base64EncodedCommand>]
[-ExecutionPolicy <ExecutionPolicy>] [-InputFormat {Text | XML}]
[-Interactive] [-MTA] [-NoExit] [-NoLogo] [-NonInteractive] [-NoProfile]
[-NoProfileLoadTime] [-OutputFormat {Text | XML}]
[-SettingsFile <filePath>] [-SSHServerMode] [-STA]
[-Version] [-WindowStyle <style>]
[-WorkingDirectory <directoryPath>]

pwsh[.exe] -h | -Help | -? | /?

PowerShell Online Help (cut because URLs are not allowed)

All parameters are case-insensitive.

---

This error can also be found in the Unity Hub logs:

{"timestamp":"2024-04-30T13:53:49.407Z","level":"error","moduleName":"UnityInstallerWindows","pid":7676,"message":"{ code: 'User did not grant permission.', message: 'The Hub does not have proper permissions to install Unity.' } The argument 'Start-Process' is not recognized as the name of a script file. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.\r\n \r\nUsage: pwsh[.exe] [-Login] [[-File] <filePath> [args]]\r\n [-Command { - | <script-block> [-args <arg-array>]\r\n | <string> [<CommandParameters>] } ]\r\n [-CommandWithArgs <string> [<CommandParameters>]\r\n [-ConfigurationName <string>] [-ConfigurationFile <filePath>]\r\n [-CustomPipeName <string>] [-EncodedCommand <Base64EncodedCommand>]\r\n [-ExecutionPolicy <ExecutionPolicy>] [-InputFormat {Text | XML}]\r\n [-Interactive] [-MTA] [-NoExit] [-NoLogo] [-NonInteractive] [-NoProfile]\r\n [-NoProfileLoadTime] [-OutputFormat {Text | XML}] \r\n [-SettingsFile <filePath>] [-SSHServerMode] [-STA] \r\n [-Version] [-WindowStyle <style>] \r\n [-WorkingDirectory <directoryPath>]\r\n\r\n pwsh[.exe] -h | -Help | -? | /?\r\n\r\nPowerShell Online Help (url)powershell-docs\r\n\r\nAll parameters are case-insensitive.\r\n"}

It's admittedly pretty funny that you can't use Unity Hub to install Unity.

Have the same problem with Windows 11 with 3.7.0 version

Also having this issue with 3.7.0

Currently having this issue with Unity hub 3.7.0
Have to download Unity from the download archive

Same error when trying to add modules to an existing Editor. Did work once restarting the Hub as Administrator.