As Numb3rs says above, and many further accounts on twitter and other social media platforms, BA seem unable to book avios tickets at the moment. BA by their response on twitter are aware of the situation and hope to have it fixed soon. In the meantime, if you then call BA, and press through the options to get to make a booking with avios you get a message saying that due to covid blah blah blah we cannot take your call right now, please try later (better than yesterday where the call simply cut off). Anyone worked a way around this?