Causality

If we cannot know that there is causality, does that mean causality does not exist?

I understand those to be two separate questions; they can be related, but needn't be.

The existence of causality is an ontological question; ontological status typically is not constrained by our capacity for knowing about it.
The possibility of knowing of causality is an epistemological question; the epistemological status typically is constrained by an agent's capacities as well as by the ontological status of the phenomenon in question. Of course, different agents have different capacities, and so epistemological status of a given phenomenon is variable among different agents.

Thank you. Your response is excellent and helpful.

Does Hume deny causality? Of just our ability to know causality?

And then, does he accept that there is some causality that can be known?

I"m not familiar enough with Hume's epistemology, but I think your questions would be a good way to approach his work. My best guess would be, that as a member of the Scottish Enlightenment, Hume would not deny causality nor would he consider it completely beyond human understanding. But that's a guess based on generalities.

The answer is no; Hume overturns both Berkeley and Locke when it comes to causality. This pair of thinkers had critically broken down the human perception of matter; but had left the concept of 'minds' alone. (Locke differs from Berkeley only in that he still allowed for 'forces' between particles). But Hume denied that there were any grounds for believing in persistent minds over time; and so he stopped taking causality (as they did) 'on trust'. Further, he denied that this Locke-ian idea of 'forces' was anything but a false construct. To Hume, causality was not supported any more since the idea of 'mind' was no longer supported; hence causality to him was a myth of the mind.

Where he does all this is 'An Enquiry Concerning Human Understanding'.