Hello! This may be due to errors in the subject line or plug-ins, try to disable all plug-ins and see again. If you customize the boot – include a plugin on one page and re-examine, so you can find the plugin that causes an error in the formation of a preview of the site. Good luck!
I presume you’re referring to WordPress dashboard > Appearance > Customize. Are you even seeing that option on the dashboard?
Go to Dashboard > Plugins and deactivate the plugins one by one.
Does the Customizer now work? If so, which plugin was at fault?
If not, go to Dashboard > Appearance > Themes and switch to one of the default WordPress themes. Does the Customizer now work? If so, your theme may be at fault.
If not, go to Dashboard > Updates and reinstall WordPress. Fixed?
If not, weep. Then give us an update, more information etc.
