Saving my nextcloud client folder to an external drive with windows 10 internal saving application doesn'twork

I want to save my client nextcloud folder to an external drive with windows 10 internal saving application.
When I want to add the nextcloud folder in the save options of windows, I can’t deal with it.
It is the same with softwares like transcend backup : I can’t select nextcloud folder.

Thank you for helping
version 3.0.2