Starting point:

Get-ClusterResource | Where-Object -Property ResourceType -EQ "SQL Server Availability Group" | Select-Object Name

Option 1: Just add -ExpandProperty to your select-object:

Get-ClusterResource | Where-Object -Property ResourceType -EQ "SQL Server Availability Group" | Select-Object -ExpandProperty Name

This will change the type to System.String and allow you to use it for your filter.

Option 2: Specify the property name which you already deteremined