Basic HA mode is having multiple restrictions and it's not possible to backup a replica node while the main node is active. To workaround the issue please make sure that in the Core console:
- both cluster nodes are protected and having exactly the same protection schedule
- replica node protection is paused
Once main node fail, the SQL cluster will switch to the replica node and it'll start working as a main node. When that happens please:
- pause the protection of the failed main node
- unpause the protection of the replica node
Reverse the process once the situation goes back to normal.
More information about SQL basic HA can be found here: https://docs.microsoft.com/en-us/sql/database-engine/availability-groups/windows/basic-availability-groups-always-on-availability-groups