Why TOC scroll is not working on my posts?

By default the GP Smooth Scroll option requires you to add the smooth-scroll class to any Links you want it to apply to. eg. Menu Items.

As you're unable to add the class to the TOC plugins links, the PHP Snippet will apply smooth-scroll to ALL links.

So you MUST:

1. Enable Smooth Scroll in the Customizer > General.
2. Add the snippet i provided.

Thank you so much David for clearing all my doubts. Smooth scrolling is working as I expected.

We can close this now.

Glad to hear that!