I’ve read this which seems relevant since I’m using xRDP.
The DISPLAY environment variable is not set. The xsel command requires the DISPLAY environment variable to be set because it interacts with the X server. You can check the value of the DISPLAY environment variable by using the echo $DISPLAY command. If the DISPLAY variable is not set, you can set it using the export DISPLAY=:0 command.
❯ echo $DISPLAY
:10.0