Cannot access the local farm. Verify that the local farm is properly configured, currently available, and that you have the appropriate permissions to access the database before trying again

I have added powershell via : Add-PsSnapin Microsoft.SharePoint.PowerShell

but when I want to run "Get-SPFarm" in powershell, got the error:

Cannot access the local farm. Verify that the local farm is properly configured, currently available, and that you have the appropriate permissions to access the database before trying again

I have installed the sharepoint 2010 and sql server 2008 r2. the account can access the db.

what happened? I'm appreciate if anybody can help me.

Awen