Footprint of SamacSys component shown as "invalid" in schematic editor

I don’t really understand your approach to learning KiCad. External stuff such as Samacsys / PCB Libraries etc, can be a nice addition, but they’re still extra’s, and not part of KiCad’s core.

A bunch of years ago, Rene Poschl wrote:

It’s written for a pretty old KiCad version (V5), and KiCad changed quite a lot in the 4 years after that (which is a good thing, lots of progress).

There probably is no universal “best” way to learn KiCad, but starting with a beginners tutorial to get familiar with how concepts are implemented is probably a good start. For example: Getting Started in KiCad | 9.0 | English | Documentation | KiCad