Just to note that I'm also experiencing this error in 2025. When I follow the steps above, this is what I get after running a terraform command like apply:

│ Error: Unsupported argument

│

│ on main.tf line 36, in resource "google_monitoring_alert_policy" "request_count_alert":

│ 36: severity = "WARNING"

│

│ An argument named "severity" is not expected here.