It would be better if you use a terminal AUR helper such as say that is shipped with 
https://discovery.endeavouros.com/aur/yay-an-aur-helper-written-in-go/2021/03/
If something goes wrong, you at least know the error message and can either do a web search or ask here at the forum.
Once you are adding several repo’s to your pacman.conf it is better to do package management from the terminal.
Add/remove software is called pamac and is developed by Manjaro and doesn’t exactly work well with Arch based distros.
More info here:
https://discovery.endeavouros.com/articles/does-endeavouros-frown-upon-gui-solutions-for-pacman/2019/11/