The BIOS doesn’t show the NVMe SSD as long as the CSM has been disabled, but nevertheless the SSD is detected by the BIOS and can be used even as bootable system drive. Please read carefully the related chapters of >this< Guide (especially the “Additional Notes” of chapter “Step 3 - Flashing of the modded BIOS” and the complete chapter “Step 4 - Installation of Win10/11 onto the NVMe SSD”).