In some cases, hosts will set a limit on the amount of memory WordPress can use, and this can cause certain issues, like the Customizer not being able to save or even load (just shows a white screen).
There’s a couple different ways to increase your memory limit.
Just ask your host – they’ll more than likely be happy to increase your memory limit.
The WordPress Codex has an article on this here.
Find your wp-config.php file in the root of your WordPress installation by using FTP.
Then add this to it: