Unable to Add Pages in Pages.app

Your problem stems partly from the way you're using Pages differently from how it would expect to be used.

Deselect all your objects. For some reason that escapes me, Pages won't let you add a new page when you have a fixed object selected. Since all your objects are fixed, that's likely to be what's contributing to your problem.

The problem may also be exacerbated by the black background on your pages... Did you create a rectangle object, fill it black, and scale it to fit the page? If so, that would preclude you from deselecting all objects because any click on the page is, at the very least, going to select this box.

In the short term, try moving or deleting the black box so that you can click on the page itself. Then you should be able to add your additional pages, then restore the black box.

Longer term, take a look at Keynote. It's far more suited for this kind of setup, and it's no harder than doing in in Pages (in fact probably easier, since it's using the application in the way it's expected) so the learning curve should be minimal. Most of the complexity in Keynote comes into the glitzy transitions and effects, which doesn't look like it's necessary here so can be eschewed.