This is surprising to me both as an avid MSC cruiser and as a TA. I book solo cabins with MSC all the time. Granted, there is sometimes a “single supplement”, but, as previously stated, they also run promos for singles. A good TA can let you know when those dates are available. Again, I’ve never not been able to book singles..
If this a group booking, I’d call MSC directly and have them facilitate it for you.