What does a Not Ready result mean?

Not Ready means the data does not satisfy applicable readiness criteria; it is not a passing test. It can follow a code clear, battery replacement or repair. The criteria depend on vehicle data, so a fixed 'drive 50 miles and it will pass' promise is not reliable.

Ask your tester or repair provider what must be checked before the next attempt. Continue only with safe, appropriate vehicle operation; do not ignore a fault to accumulate driving time.

Official source: CARB readiness information.