In my case the cause was my PowerShell profile script (Documents\WindowsPowerShell\Microsoft.PowerShell_profile.ps1) throwing errors on startup.
If you’ve installed the newer PowerShell 7 and use features from it in your profile, note that Waves Central still runs the old built-in Windows PowerShell 5.1 and if your profile uses newer syntax, 5.1 basically throws errors on startup.
Waves Central apparently interprets any error output from PowerShell as “can’t access PowerShell”.
Quick test: Open plain Windows PowerShell (%SystemRoot%\system32\WindowsPowerShell\v1.0\powershell.exe) and if you see any sort of red errors at startup, get those fixed and Waves Central will most likely work.