Why is my car still not starting?

Battery tested good, starter tested good, fuses all good, new terminal connections, cleaned grounds, what could be the issue??? I get a check oil, check engine, and check battery light in my car. Dash lights will lightly flicker when I turn the key and it 'cranks.' Ran fine earlier this week. No prior issues. Volts on battery read 12.6 V doing nothing and 11.3-11.9 V when trying to start