Baffling inability to create trade route

Never seen this one before. I can see two cities of Aksum, a distant land civ.

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The city of Aksum is not on the "available" list, but its immediate neighbor Adulis is.

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The weird thing is, Aksum isn't even on the UNAVAILABLE list! Any ideas? I've ruled out the usual stuff like range and capacity,and both cities are on the same continent.