Welcome WhiskyGolf!
Probably the video uses another version of LibreCAD or the author did not explain all the steps.
So it helps to provide more information, e.g. a link to the video in question, your LibreCAD version (Help-About) and maybe operating system.
A guess is, that the video shows a rectangle built from single line entities and your rectangle is a polyline.
The polyline is an entity container and you can select only the whole container, not a single line or arc.
Use Explode from Modify tool menu to explode the rectangle, then each side can be selected separately.
Armin
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