Is the reason you want to use the PSExec activity because the remote client doesn't have the CommVault software installed on it? Since if the remote client does have the CommVault software installed then you can just use the ExecuteCommand or ExecuteScript activities.
Can you try executing the paexec.exe command manually from a command promt on the workflow engine client to see if that is working? The location of this executable is in the <installation path>\WFEngine folder. Also executing the command manually will allow you to enable debugging to see more information to troubleshoot.
A couple of reasons why you can get this error is:
1. If the workflow engine client and remote client or in the same domain, then you'll need to enable useImpersonation option on the PSExec activity input, otherwise you'll need this option off.
2. You need to disable UAC on the client you need to execute the command on.