SQL Server AAG: failed to obtain cluster information. either the specified instance of SQL Server is not running on a windows server failover cluster (WSFC) node, or the user lacks sysadmin permissions on the SQL Server instance to obtain the cluster information.

You get the following error when trying to add a node to an AlwaysOn Availability Group.

failed to obtain cluster information. either the specified instance of SQL Server is not running on a windows server failover cluster (WSFC) node, or the user lacks sysadmin permissions on the SQL Server instance to obtain the cluster information.

The first things to check are the following;

1) check always-on is enabled in SQL configuration manager or run SELECT SERVERPROPERTY (‘IsHadrEnabled’);
2) check you are local admin on all nodes 
3) check cluster permission by right click on properties of the  windows cluster in fail-over cluster  manager

There is also a qwerk with SQL Server, where if you Enable the AlwaysOn Availability Group setting before the node is added to the cluster then you get this error when trying to add the node to the AAG. You need to Add the node to the WSFC THEN Enable this option. If you’re in doubt then this option can be unchecked then checked again, how ever this will need a SQL Server Service restart.