Customize Slot, Customize Section in builder do not work

Your software
My Mautic version is:2.15.3
My PHP version is: 7.2.27

Your problem
My problem is:
What is the purpose ofr Customize Slot and Customize Section in the builder? Clicking in the box does nothing and they don’t seem to work.

These errors are showing in the log:

Steps I have tried to fix the problem: