@User a PDK_ROOT environment variable is optional and tells xschem(and other EDA tools) where the skywater stuff is installed. If this variable is not set xschem does some guesses (in home dir, and /usr/local) and if it finds a valid install it will use that one. The message you get about missing PDK_ROOT is just a warning, not an error. Another suggestion, do not launch xschem in the background (

xschem &

), use

xschem -b &

. Xschem uses the terminal as a command shell, the -b option tells xschem to detach from the terminal and then you can launch it in the background.