Garmin Drivesmart 55

Completely wiping the Australia / New Zealand maps won't help you install North America maps, each unit is setup from new to accept either no maps (this is rare), time limited updates from one area (also rare) such as North America, Europe or Australasia or lifetime maps from one area (this is normal), your unit most likely has lifetime Australia maps, this is setup by adding small files to your unit which tells download programs what downloads are available and which are not

If you purchased your unit in the North America then I would say that the seller must tell you that it does not have NA maps, if you was not told this I think you should get a refund on your purchase

You won't be able to download NA maps to this unit even if you delete all current maps, you can buy NA maps from garmin and these may come with time limited or lifetime updated BUT they are not cheap

If you can't get a refund and don't want to purchase NA maps you can download free maps, see link HERE