In Civ6, what can you demand from a weaker civ?

There are a lot of factors that go into what demands opposing Civs will give in to. Your military strength, their fear of you (have you killed many other people/cities), what do they have in comparison to you and the world's opinion of you (along with more factors that affect the math behind it).

You should be able to demand gold from opposing Civs, I am not sure why it is not showing as an option for you. It might just be a bit bugged out unfortunately.

In terms of other things you can demand, it can be virtually anything; cities (besides capital), resources (luxury or strategic), gold/gpt, etc.

As for why they likely won't accept anything: if they are likely to just die anyway, where they only have one city and they likely have little to no military, then the AI is basically like; well it's pointless and it wouldn't stop you, so no.

Things you can do to increase chances of demands from being successful in the future: Not only have a much higher military strength than them, but have those troops near their borders.