Samsung note 20 and the back cover of the phone is not really level.

Hi

If it is "new", take it back to where you bought it from and get it replaced under warranty.

New phones should be perfect out of the box.

If it is just "new" to you and is a used phone you will have to open it and check if the battery is swollen or not. Here's the ifixit Samsung Galaxy Note20 배터리 교체 guide that will help.

You may have to check what the seller of the phone stated regarding the condition of the phone when it was advertised for sale, to know if you need to contact them if you find that it is a swollen battery. They should have mentioned this in the description of the phone's condition. Negotiate a new price for the phone if it needs repair that wasn't mentioned by the seller or if the condition was stated "as is" etc and there wasn't enough good images to see all sides of it