I haven't had this situation, but I think you need to sign a peace treaty with the AI that you conquered it from.

I'm not denying that this may feel annoying.  If you want that a unit does nothing for one turn and you still want to end the turn, you can give a "SKIP" order to the unit.  Probably, the game developers should make it possible to give a "SKIP" order to a city if you want the city to do nothing for a turn.

That said, a temporary workaround might be to go to the Game Menu (pressing the Escape key), then to the "Settings", then to "UI", then set the "Enable Mandatories" configuration option to "OFF".  After that, you might be able to end the turn - i'm not completely sure because it didn't reproduce the problem.

You can re-enable the "Enable Mandatories" option later after the situation with this particular city has been resolved.

I tried disabling mandatories and it didn't help. It genuinely seems like a bug to me, as the UI tells me to pick the outpost with the most resources to turn into a city. I had an outpost with 3x as many resources (I'm new to the game, first one wasn't great) that was going to finish being built in one turn, but I'm not allowed to exercise my new information and informed judgement and that is a real shame.

Same issue, forcing an arbitrary first outpost to be first when I'm a turn away from building an actually good city is incredibly frustrating