Backup DB's on High Availability issue

I have an issue with backing up the DB's from cluster server that have 2 nodes on him.

I have a Cluster server with High Availability (for Active/Active cluster) and I trying to take a full backup from him but it says that :

The selected backup type is not supported on a "Always on secondary replica"...

And sending me to Microsoft webpage : http://go.microsoft.com/fwlink/?LinkId=208213 

Active Secondary's: Backup on Secondary Replicas.

All I want to do is to backup the primary node with Full Backup type on nightly basis and shrink the log file . (without un-synchronizing every day)

Thank you