I'm trying to backup my availability group in SQL Server 2016 using DPM 2016, but after I add the cluster in DPM, availability group show nothing to back up. I've set every secondary node to be readable secondary and add sysadmin role to \SYSTEM user. How can I fix this?
If you have added the cluster, as in the WSFC computer name, to DPM then this may not be correct according to the MS documentation and may explain why you have nothing to backup - https://docs.microsoft.com/en-us/system-center/dpm/back-up-sql-server?view=sc-dpm-1807
Try adding the Availability Group Replicas individually. DPM should detect the AG configuration automatically and, provided all replicas are readable, it should backup up according to the AG backup preferences.