It seems that :component-did-mount does not return a DOM node. You need to call (reagent/dom-node comp) on the component it returns to get the DOM node.
But why are you even adding the click handler imperatively in a callback? That is usually not the way to do it in reagent (or React for that matter). Instead, do it declaratively as part of the attribute map in the render function by setting the :on-click key to your on-click handler. It’s less code, much more idiomatic, clearer, and less prone to weird bugs.