Test-DbaDiskAlignment Not Working
Check out this post, do you meet the minimum requirements? Is remoting enabled?
https://dbatools.io/install/

Otherwise run it with the verbose flag and let us know what you get:

Test-DbaDiskAlignment -ComputerName Test_Server -Verbose


Also what version of both the dbatools module and powershell are you using?

Powershell version:

$PSVersionTable


dbatools version:

Get-Module dbatools