If you are allies with the Empire, you can burn diplomatic favor to call a caravan just the same as any other faction.
Unfortunately, you can call a caravan when you don't have a title, but to actually interact with that trader you need a title (I'm guessing Knight / Dame, but it doesn't say explicitly).
Seems unfair to let the player burn diplomatic relations on something they can't use.
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Also, THIS IS NOT PART OF THE BUG, but...
Why would you make Count Petronas a caravan guard!?
I already said in the last bug report (https://ludeon.com/forums/index.php?topic=50701.0) he's an 81-year-old man with chronic health issues, who has no place in a fight. Leave the man alone, Tynan!!
The caravan calling request issue was fixed, thanks for reporting.