You get that message when you try to set the RTH alt to greater or equal to the Max Flight Alt, which your screen shows at zero. BTDT. How you got MFA to 0 - IDK? They only show 0 if you are not connected to the P3 / RC as the settings are held in the P3 and read from there to display and then written there when changed. I don't think a copy is held in your iDevice / Android memory as you could be using it for multiple RC's / birds.
If after connecting to the RC & P3 and not being able to change, then I'd suggest you sync your flights, delete the app, which will delete the data, re-install and re-sync your flights from the DJI server.
And of course this could be one more bug in 3.0.0.