Similar happened to myself when travelling ALC > LGW recently. According to BA.com ALC isn't listed as supporting mobile boarding passes:
https://www.britishairways.com/en-gb...VINF2ba_mobile
You can check in normally online, but both the mobile app won't allow you to retrieve the mobile boarding pass. The message I got on mine was that my travel documents were either expired, not entered or needed checked at the airport. I guess that's some kind of default error message when using a mobile boarding pass isn't possible.
It's just a case of going to check in as normal and them issuing a paper boarding pass. A bit of a pain if you're HBO.
What I did notice is that once the paper boarding pass has been issued at the check in desk you can then retrieve it on your mobile and use that for lounge acccess, boarding etc.