The Surface Painter can only be used when the 3D window is active. So switch to the 3D, and the item will be enabled.
As for your AC18 file migrated to AC19 sounds like you need to check your Work Environment settings (because it's probably using your AC18 WE where the function didn't exist) and add it to the your menu (via Commands Layout Scheme > Menus > "Show/Hide Surface Painter") or you can use the AC19 Work Environment for the AC18 file you migrated.
James Badcock
Graphisoft Senior Product Manager