Clear-FASTSearchContentCollection : Failed to communicate with the WCF service.

According to this title error when clear fast search index using PowerShell command.
Clear-FASTSearchContentCollection -Name sp
and I got this error
Failed to communicate with the WCF service.
then i changed user account to run this command by Shift + Right Click on FAST Search

Then log on as FAST service account to run this command instead of current account.

Hope this helps.