Make sure you are on a later or latest BIOS, NVMe/PCIe boot was added later on to many Asus Z97 boards though it is quite tricky to get it working. IIRC you need to boot from another drive to install a driver in order to make the drive both visible and accessible. I owned an Asus Z97-P and used both M.2 SATA and NVMe boot drives, both were bootable when I had them installed (I upgraded to the NVMe drive after they enabled boot). As an aside an M key socket can accept both M key drives AND B+M key drives. Almost all SATA M.2 drives are B+M key so if you can find a SATA M.2 drive it will work as a drop in. They are still around if you look.
Edit: To add, the M.2 slot is often disabled in situations where enough SATA ports or PCIe slots are in use. It will depend on your specific board and what onboard devices you are using. Check your manual as well as descriptions in BIOS releases for any changes. Now all that said, if your current drive is B+M (2 slots on the edge connector) I'd give it a try again for fun but otherwise follow the advise of the others, there's SATA 2.5" drives up to 2TB readily available.