You’re trying to use the Healing Brush tool, but don’t see results. What is the problem?

You forgot to Shift-click to set the source point for the tool
You forgot to Alt-click (Windows) or Option-click (Mac) to set the source point for the tool
You forgot to double-click to set the source point for the tool
You forgot to Ctrl-click (Windows) or Command-click (Mac) to set the source point for the tool