The nice thing about powershell commands is you can use the tab key to scroll through your options. For instance if you type move – i and then hit your tab key each time you click your tab it will show you another command starting with the letter i. I tried this on my exchange server and it did not show me a move -imapmailbox command, so it sounds like the error you got is correct. It does not exist.
I made some progress… I was using the Exchange shell and realised I needed to use the Transporter Suite shell. The cmdlet is available in there and working.
I am now fiddling with the actual automation… Steep learning curve…