As mentioned above, the seat map never tells the whole story, especially this far in advance. However ExpertFlyer is also showing this fare bucket availability for your flight:
So for whatever reason, even if it's just some intern screwing up, they're not currently selling any business class tickets for that flight. Looking at other flights that week, Mon&Fri have considerable availability across all non-I buckets in business, while Tue&Thu have none. All the while economy tickets in all non-G buckets are available on all four flights that week. There's no way to force them to open availability in a bucket.

I would dare speculate there's a possibility they might be considering reducing service from 4pw to 2pw due to the current situation/lower demand than expected, and preemptively don't want having to reaccommodate additional business passengers if they end up deciding to reduce service.

As noted above, there is availability on the 15th of May if that happens to be of any use to you.