Otherwise, there are mounting kits for AM4 that include a backplane. You should also be able to use these for the Noctua cooler.
--> be quiet! AM4 Mounting Kit
--> EKWB AM4 Mounting Kit for Supremacy Evo

Since the plate is part of the mainboard, you have been supplied with a defective mainboard.
You will have to contact the supplier and request a replacement.

The backplate should be pre-installed on the mainboard.