I would like to customize the layout of single products because I would like the short description under the add to cart button and variables. If I try to create a new layout and choose the Predefined layouts and then click create the layout opens a blank page. How can I start from a Predefined layouts?
Please deactivate all the 3rd party plugins and activate only theme-required plugins on the site and then check the issue. I am sure your issue will be solved. Then Activate the 3rd party plugins one by one and check which plugin is creating the issue for you.
Otherwise, if the issue still exists then keep the 3rd party plugins deactivated and let me know I will check and give you a possible solution.
Please confirm the permission for plugins deactivation, and switch to the parent/default theme. As soon as we complete the testing of this plugin we will enable it all back, You would better make the full backup of your site.