If you're having trouble getting your dialogue UI to response to mouse clicks, here are some things to check:
Are there any errors or warnings in the Console window? (Make sure the Console window isn't filtering out errors or warnings.)
Verify that the scene's EventSystem looks correct. It should have an enabled StandaloneInputModulecomponent, or a UnityUIInputModule component if you're using the Input System package.
Also check that the Dialogue Manager's Canvas has an enabled GraphicRaycaster component.
Finally, keep an Inspector view on the EventSystem while you're playing. At runtime, the EventSystem will report what UI element the mouse cursor currently detects. Maybe some other UI has an invisible element that's blocking the mouse from detecting the Dialogue Manager's UI elements.