I have downloaded 2024 Pro and it will not allow me to save to earlier versions. Why?
That option was removed. See the release notes. The SketchUp 2024 file can be opened in SketchUp 2021 and later without needing to be backsaved.
There are several extensions that can save as versions eariler than 2021. Look in the Extension Warehouse. Also see this thread.
Please update your forum profile. It says you are still using 2017 Make.