12 TB Western Digital hard drive

I’m assuming this HDD is being mounted internal; in one of the four main HDD bays, or in a retrofit bay in one of the two optical drive bays.

Try that HDD in a different enclosure with a different SATA controller or different server; preferably one with 6 Gbps support. That might be a SAS/SATA 6 Gbps controller in the Mac Pro. See if the HDD works there.

Early Intel Mac Pro and its contemporary Intel Xserve servers can tend to be fussy about their HDD storage. Mac Pro 2012 wants / needs SATA 2.0 / SATA II /SATA 3 Gbps HDDs, or slower, and this is a 6 Gbps drive, which means you’re dependent on how well this HDD falls back to SATA 2.0 or SATA 1.0 connections that the Mac pro will expect.

If you decide to return this, OWC has HDDs that should work here, but confirm with them before buying. https://eshop.macsales.com/shop/ssd/owc/mac-pro/2009-2012 (scroll way down) (I’m using WD UltraStar in NAS, and these prices are reasonable.)

Some Amazon marketplace sellers are seemingly prone to scams and fraud. Those concerns aside, this wouldn’t be the first dead-on-arrival new or refurb HDD, that even from a reputable seller.

I would return the drive since it was not properly packaged assuming this was a bare internal drive and not a retail external enclosure? If it is an external drive, then you've been ripped off since an external drive should at least have the retail packaging. I suggest purchasing drives from a more reputable vendor than Amazon or one of the Amazon marketplace vendors many of whom have very low standards.

Keep in mind with a bare internal drive some SATA Adapters, drive docks, and enclosures may not be compatible with a 12TB drive. I'm not sure whether a Mac Pro is able to work with an internal 12TB drive since they were not available at the time the Mac Pro was supported by Apple. You can try connecting the drive externally using a compatible adapter/dock/enclosure to see if the drive works which can tell you if their is a compatibility issue with the internal controller of the Mac Pro.

Try to format the whole device (NOT just volumes under it, so set Disk Utility > View > Show all devices) first as FAT (Master Boot Record MBR) and then as MacOS Extended (GUID). If that does not succeed after a reboot and a few tries, I'd return the drive.