: Trimble (the developer) generally only supports and provides downloads for the current and two previous versions of SketchUp Pro.
SketchUp Pro has always been a paid professional product. While many sites claim to offer "free" downloads of version 18.0, these are often unofficial and carry significant security risks.
If you don't have a legacy license but want to use SketchUp without paying, consider these official alternatives: Where can I download Sketchup Pro 2018?