GetNode() works on a path system based on which node calls it. Your code is attempting to search for a child of your character named UI instead of the UI itself. I’m unsure if it’s different for C#, but the path for the UI in GDscript would be “/root/Test/UI/CombatUI” assuming Test is the scene’s root node.
Using the Unique Name with GetNode can cause issues when instanced scenes are involved (I assume the characters are instanced scenes). If you still want to use the Unique Name method instead of using the direct path, you can likely better fix your issue with this page on the Godot Docs.