This site is being deprecated.
Please see the official X‑Plane Support page for help.
My first suggestion would be to run the installer and pick the update option to see if any default files are missing or broken. Also make sure X-Plane is not installed to the Applications folder which can cause permissions issues.
IF neither of these things helps, attach a copy of the installer log.txt or log.txt.
This site is no longer being actively maintained.