Using PMP Pro to Protect "Portfolio" posts

First of all, you shouldn’t be trying to modify PHP in the PMPro plugin, or else you’ll just lose those changes the next time you update. Instead, modify through functions.php or a custom-built plugin that houses all your PMPro changes.

I’m not familiar with the Portfolio CPT that you’re using but Jason’s post here should point you in the right direction to enable PMPro protection on any Custom Post Type:

https://www.ads-software.com/support/topic/plugin-paid-memberships-pro-custom-post-types-taxonomies?replies=4

(Again, it is NOT recommended to modify the core plugin files directly so I am NOT suggesting you make the modifications suggested by user wtoalabi in that thread.)

Here’s an add-on that may make all of this simpler – I have not investigated the details but hopefully one of these links will help you get where you need to go.

https://github.com/strangerstudios/pmpro-cpt

Separately: I could not get to the page https://www.cliffswain.com/portfolio-view/welcome/ – it redirects me to the membership sign-up page which I assume is the desired behavior?