You subscribed to Cloudflare Pro, which gives you access to a few more things in WAF, like OWASP Core Ruleset, Cloudflare’s full managed ruleset, zone lockdown, etc. That page is just a kind of confusing list of what Cloudflare Plans get access to features belonging to that product.
I believe the plan the message is referring to is Enterprise WAF Advanced, which would be an addon Enterprise. Enterprise gets everything piecemeal (as they request), so it looks like base Enterprise does not have access to that, if you were Enterprise, you’d need WAF Advanced addon (and Enterprise is not going to be cheaper then Business).
You could most likely use a custom Worker to do what you are wanting, although you would need to pay for the invocation cost on each request. Snippets are coming soon: Cloudflare Snippets is now available in alpha, and would fit this better & make this possible on lower plans without the invocation cost, but not released yet.