Here is some culprit you can’t save your customizer.
1. Lack of Memory
Several method to add memory limit
Add following line to your wp-config.php
define( 'WP_MEMORY_LIMIT', '256M' ); define( 'WP_MAX_MEMORY_LIMIT', '512M' );
Add following line to your .htaccess file can solve this issue as well:
php_value memory_limit 512M
Note : Sometimes, your hosting provider has been limited this value from their server configuration, so you need to ask them to adjust their settings for you.\
If you’re running unreliable shared hosting ( tons of websites under one server ) you may experience this one.
2. Error 503
If you are encountering a 403, 500, or 503 error when trying to view the Customizer, we have found that this is typically due to security settings or permissions errors with the server. For this particular situation, you will need to contact your hosting provider and ask them to ensure that your file permissions are set correctly. Additionally, this could be related to a ModSecurity rule being triggered due to a combination of keywords used in the core WordPress files for the Customizer. This should be able to be remedied by writing into the support team at your host and asking them to whitelist the ModSecurity rules on your domain.
3. Clearing Your Cache
4. Disable 3rd Party Plugins
Deactivate all plugins, then try to access your Customizer.
5. Make sure there aren’t any Debug Errors
Enable WP_Debug from your wp-config then check error message that you see.