I've never used the IDE : I always use the Web Editor

How can I put the LCDWIKI GUI Library into the folder “Libraries” as I do not know where it lies ??
Thanks for your help

The README is refering to Arduino IDE at this point, not PlatformIO.

Place the LCDWIKI_GUI library folder your /Libraries/ folder. You may need to create the Libraries subfolder if its your first library. Restart the IDE.

Please use the library manager to add a library to your PlatformIO project. We have this library in the registry.