When is the flight? It is probably due to Comair suspending flights (from 2 July to 31 August 2021). BA probably can't see the booking system or it is a technical detail of how Comair actually suspended their operations. It might be that Comair start running again sooner, now that the leisure travel ban from Gauteng has been lifted. But equally I wouldn't expect BA to be running flights from JNB until the 'red list' is changed. Qatar seems to be the most reliable alternative.