SQL Server 2016 Availability Group on DPM 2016
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.