I had an Imperial Trade Ship signal me to let me know it was available. My colony is allied with the Empire and I have a Praetor as my highest rank pawn.
I tried to have the Praetor call the ship, but it's just greyed out. No indication why there's a problem. I clicked it anyway, but it just gives the unhappy refusal audio sound. No other signs or signals as to what the issue is. I spent 15 minutes thinking it was a bug to report.
You need to be a Baron to call an Imperial Trade Ship, but it never says that anywhere. It really should.
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I realized as I was writing this that I do technically have a Baron staying with me as a guest, so it could be that triggered the Imperial Trader to visit. If you've never seen the Baron title gained message (which I haven't, yet), it's unlikely you'll ever read in-game that Orbital trading is a Baron-level privilege. So having guests trigger the orbital visit is probably a bad idea unless the privileges are made clearer.
It's not a bug, but I agree it needs to be made more clear.
Thank you carbon, issue resolved.