So I contacted support about not being able to access two dungeons from Legion:
A Court of Stars
The Arcway
I had the opening quests for both of these, and there was no option to select them from Dungeon Finder. I even tried going to the dungeon location and every time I tried to enter the dungeon, it would kick me back out and not let me in.
One of my friends could queue with me in party so long as they were party leader, but it did not make them queue-able for me after. I even leveled another character and they did not appear. This was both in and out of Chromie Time.
Support suggested I submit a bug report.
It likely gives you an error on your screen or General chat window when you’re kicked out after trying to enter. What is it?
I believe both of these dungeons were for max level Legion characters (e.g. not Legion leveling dungeons) so maybe they still have a minimum level requirement that your character isn’t meeting yet?
From everything I can tell you’re required to be both level 45 and have actually unlocked the dungeons by completing a certain amount of the Suramar questline - specifically you must have completed the Friends with Benefits quest, which is only available after 8,000/12,000 Honored reputation.
This is in addition to ensuring you’re trying to enter on Heroic or Mythic difficulty. Neither of these dungeons has a Normal difficulty mode.