The slots do not determine if the drive is SATA or NVMe, the slots determine if it's PCIe x2 or PCIe x4. SATA works with either x2 or x4, NVMe requires x4. Your computer should have had the socket that could be either x2 or x4, but the chipset supports only x2, so you need a SATA SSD. It will fit the socket and work. If you look you will see that a B+M card fits into a B slot, an M slot or a B+M slot. What you cannot use is a B slot only card with the M slot connector you have.
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