So this software is asking me to load a 2015 or 16 Sketchup File, instead of my 2021 file. Does this mean that 3D Exchange only accepts 6 year old outdated versions files  ???

It's silly on the part of Trimble to introduce incompatibility, as there is essentially hardly any difference between the latest Sketchup files and the ones accepted by 3DXchange.

It's even more silly on Trimble's part to exclude saving in an earlier version and to make the free desktop version no longer available.

You can use the free https://pcon-planner.com/en/download/ to convert your model to an earlier Sketchup version.

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