xauth's location needs to be added to the environment variable, PATH. When you are able to execute a program on the CLI without being in the directory for that program, that is PATH at work. To view PATH:
Now you need to locate xauth so you can put that directory in the comma delimited PATH:
whereis xauth ,or, which xauth
Found the directory? Good, add it to path:
PATH=$PATH:/path/to/dir-containing-xauth && export PATH
That should have modified PATH, do it as root too for good measure.