I had been following this tutorial

https://www.digitalocean.com/community/tutorials/how-to-configure-secure-updates-and-installations-in-wordpress-on-ubuntu

and I got it working, but now I can't customize my themes.

After troubleshooting for several hours, I'm thinking it might be a permission issue but I can't put my finger on it.

Does anyone know remotely what might be causing this?

My issue is very similar to this one
https://siteorigin.com/thread/theme-customize/
but my wordpress address (url) and site address (url) are exactly the same (which was the recommended fix)

Here is an image of the error.

Please let me know if I can provide any more information.

http://oi60.tinypic.com/9uxhl4.jpg