There's no such thing as an NVMe slot, just M.2 slots that have PCIe lanes. NVMe is a protocol that runs on top of PCIe, and is something that host systems supp

There's no such thing as an NVMe slot, just M.2 slots that have PCIe lanes. NVMe is a protocol that runs on top of PCIe, and is something that host systems support at the software level, not in hardware. (Firmware support for NVMe is necessary to boot off NVMe SSDs, but the Pi doesn't have that and must boot off eMMC or SD cards.)