Went to enable Client-Side Security Advanced (formerly Page Shield) on a Cloudflare zone and it wasn’t listed anywhere under the available Add-ons in the dashboard.

Logged a ticket with Cloudflare support. Turns out it’s not you, it’s them:

I have checked with our engineering team and confirmed that the self-serve purchase option for Client-Side Security Advanced is temporarily disabled in the dashboard while we make some improvements to the billing system. This is why you are not seeing it listed under your available add-ons.

The fix is to go through Cloudflare Sales, who can add it to the account manually. Reply to the support ticket saying you want to purchase Client-Side Security Advanced and they’ll put you in touch with Sales.