Am I correct in assuming that your playbook and serverconfigs/debian12/etc/aliases are stored in the same repository on the SCM?
If so you do not need to specify the file as an absolute path. Simply use relative path.
If you want to specify it as an absolute path anyway, you can probably specify /runner/project/serverconfigs/debian12/etc/aliases.
The playbook runs in a different pod that is neither web nor task, called the automation job pod.
This pod will not have /var/lib/awx/projects mounted, but will use the project directory otherwise transferred from task pod to /runner/project.
Thus, the paths to files under your project directory will be under /runner/project.