Hi Joanne,
In this case I would reccomend you to install your Theme manually via FTP.
1. Download the latest copy of X and upload it to your themes folder via FTP or CPanel(in case you have it)
2. Latest version of Cornerstone will be installed automatically when you head over X Addons section. Now you try validating your X theme again and then try to install any other extension and it should function properly.
https://codex.wordpress.org/Using_Themes#Adding_New_Themes
Let us know how it goes.
Thanks!