You will need a M.2 NVMe SSD.

Not all physical sizes fit in the machine! My unit came with a 2242 affixed with an adapter, and can take up to a 2280 at the largest.

Anything in between those in size will require you to buy an adapter, so I recommend sticking to 2280.

Alternatively; you can open the machine up first, and then check what size you already have and buy a matching size. If you go this route, you should familiarize yourself with SSD sizes and types first.