Hello PHYO AUNG KYAW,
Thanks for your question.
THE Get-MgUser cmdlet expects and object after.
The correct syntax for Get-MgUser is
Get-MgUser -UserId '<id>' |
see examples here: https://learn.microsoft.com/en-us/powershell/module/microsoft.graph.users/get-mguser?view=graph-powershell-1.0#example-2-get-a-user-by-id
This is same for New-MgUser:
See usage here: https://learn.microsoft.com/en-us/powershell/module/microsoft.graph.users/new-mguser?view=graph-powershell-1.0#example-1-create-a-new-user
Please let me know if you have further questions**
You can mark it 'Accept Answer' if this helped.