Ok please try following to resolve this issue :
1. You wordpress is outdated .I checked your site but could not able to do so .It seems like you have disabled wordpress updates .You need to update your wordpress to latest version as all the plugins code including woocommerce and buddypress is updated according to latest version wp. This may be the reason why the customize is not working in your site .
2. Second thing is that your wplms modern theme is outdated . You need to update your wplms modern theme to latest version link given in this tutorial :
https://wplms.io/support/knowledge-base/modern-theme/
3. Third is please try deactivating all your third party plugins at once after updating child theme and wordpress and then clear the cache and then check if the issue appears or not .
Let me know if above helps .