@Alzrag Right click a node you want to export, and click on "mark for export" (in your case, do it on the "erosion" node). Then on the right side bar, switch to the "Build" tab. Your erosion node should appear there. Set your properties, check the build location template on the bottom (I'd suggest templates starting with "file location", then your save file location will serve as root directory). When you're ready, click the button named "start build". You will find your heightmap and other masks from erosion node saved in the build location you set earlier.
Hope this helps!