I got this the other week. Do you get a message saying 'Oops something went wrong - try again in 48 hours' or something like that?
Log into your My02 account on a computer web browser.(not the 02 app). Make sure your number is properly assigned to your My02 account, make sure everything is set up ok while you in there, then log back out. Wait the next day and try logging back into your My02 Android app. It should work.
If that doesn't work and the same message keeps coming up then you need to call 02 Customer Services.
😃