Problem:
ConvertFrom-String is not available on PowerShell Core Edition.
Reason:
It seems by design. According to the source of PowerShell on GitHub, ConvertFrom-String is exported for Desktop Edition, but not exported for Core Edition for Windows environment or Core Edition for Unix environment.
Solution:
None. Instead of ConvertFrom-String, use another function like Select-String -Pattern, -match etc.
Note:
Contents of Default-Aliases.Tests.ps1 is useful to take a brief look at available typical cmdlets for Desktop Edition, Core Edition on Windows environment, and Core Edition for Unix environment.
