Hi ,

About Customizing Portal, please note that prior to version 21R1 unfortunately you cannot open Customization Projects in Portal. And the way of customizing Portal in these older versions can be a bit tedious.

Unfortunately creating Customizations on Portal is not supported at this moment. (on those versions)
Creating Customization on Portal is supported in version 21R1 (and higher).

If your instance is lower that 21R1, then As workaround: create a customization for portal using ERP site, then publish this customization on a portal.  You can try the following approach, see below:

-You could  copy the Portal (all pages and DLL) to your regular site (your dev instance - NON Portal site ).
-Then you will be able to Customize those pages normally with a Customization Project.
-Then you can export the Customization Project from your regular site (your dev instance - NON Portal site ), import it on your Portal site and publish it.

Please remind to test your Customization on a Dev/Test instance first.

After testing then you could upload Customization to Production environment and publish it.

Please also review this other Portal related post: