Have you been able to customize in the past? Are you only not able to use the Customizer, or are there other issues as well?
There are a few things that you can try:
- Switch back to the Twenty Fifteen theme and disable all of your plugins. If that doesn’t fix the problem, try reinstalling WordPress, and if that still doesn’t work, contact your hosting company. Otherwise, move on to step 2.
- Switch back to your chosen theme. If that causes the issues to come back, the problem is with your theme. If not, move on to step 3.
- Reactivate your plugins one at a time, and check for problems after each one. If the problems reoccur after activating one of the plugins, the problem is with the plugin.
