Are you attempting to add personal assets or modifications to the library that is shipped with KiCad? If so, then this explains why you can not do that on Linux. The library that is shipped with KiCad is put into a directory where you need root access rights to modify its content.
This is done on purpose to protect you the user from yourself! Personal assets and personal modifications should really be done only in personal libraries that you place somewhere where you have access rights (example in a subfolder of your users “documents” folder). If you modify the libraries shipped with KiCad then the next time you update you will lose these modifications (and therefore your personal assets)
Further reading:
- Creating a new footprint library (explains how you would make a personal library AND still contains a section on how it is done in 5.0.x and earlier)
- Tutorial: How to make a footprint in KiCad 5.1.x? (This tutorial is for 5.1.x and later! 5.0.x has a different footprint editor!)
- Library management in KiCad version 5 (General information is valid for basically every KiCad version after version 4 but the interface screenshots are from version 5.1.x which is different from earlier versions)