During configuration of FAST Search for SharePoint, you will need to use PowerShell to finalize your configuration. One common error you might receive is the one below. This error occurs when you execute any PowerShell command for FS4SP.
Failed to communicate with the WCF service.
This error can occur for a number of reason such as the FAST service is unavailable or the services aren’t started. However, a really common cause is due to lack of permissions. In this case, I executed the script with another administrative account I had. It would be nice if the error just said “access denied” or something like that. To correct this, you have two options. You can either use the Run as different user menu item and use the FAST user account specified during configuration when running the FAST Search Server 2010 for SharePoint management shell. Your other option is to add the account in question to the FASTSearchAdministrators local security group on your FAST Search for SharePoint servers. Here’s an example of how mine is set up.
After you add the user to this group, you will likely have to reboot for it to take effect. Once you do, run PowerShell again and try your command. If all works successfully, the command should work.
Hopefully this works for you.