I suspect something other than that is the cause of the error. If possible, would you post a StudioLog? If you cannot, I would look there and see what messages are being reported. Since you can build the solution, the issue has to be with the communication to Robot Manager or the Package Server and likely not because of a missing DLL.
This has been resolved. The root cause of the issue was that the developer had included a version number with decimals in the solution name. Once we removed that, the solution published to the deployment portal without incident and we no longer received the "Could not locate the assembly "Interop.FREngine" version = 18.104.22.168" error.
Would have been easier if the error message actually pointed us to the invalid solution name instead of sending us on a needle in the hay-stack search.