You're in the original free Google Apps for Domains program, grandfathered. As the product has evolved, these grandfathered accounts have come along for the ride, at times with new G Suite features disabled behind a "try our paid plan for one month free" button. I don't know how many of these are left (I have one too), but they seem to occupy a weird product nether space that Google probably sees as a minor annoyance.

my company is permanently stuck at ten email accounts for this very reason.