Why does the Friedmann Equation have an incompatibility with General Relativity?

Wouldn't the curvature of space just add an additional parameter to the Friedmann Equation, that can potentially resolve to 0, to account for the Newtonian model being spot on, while still allowing for our actual observed space-time fuckery?

Am I applying brainlet logic to a much more complex problem? I feel like I'm not wrong though.