Windows 7 is missing the NVMe drivers to recognize the SSDs. You have to manually add NVMe drivers to your Windows 7 installation medium (editing the install.wim and boot.wim).
These links might help:
Follow this procedure:
1. Download Intel driver for NVMe SSD from the following link:
3. Copy Samsung NVMe Driver directory to your USB stick with WIN7 installation.
4. During Win 7 installation, while windows will inform you that the driver is missing, browse to the Samsung NVMe Driver directory, windows will install the NVMe driver.
5. Continue with installation.
This is Intel NVMe driver that is more suitable for you: