"Cannot open database version 706. Upgrade the database to the latest version" - when trying to use availability database for read-only access
What do you suggest on above ?
I need to keep primary SQL server replica as 2012 for now, and I need to make &gt;async replica (SQL server 2016) to be available for read-only access

There is nothing you can do, this is a known item:

During a version upgrade, readable secondaries cannot be read after an
upgrade of the readable secondary and before either the primary
replica is failed over to an upgraded secondary or the primary replica
is upgraded.
https://docs.microsoft.com/en-us/sql/database-engine/availability-groups/windows/upgrading-always-on-availability-group-replica-instances