have you used this DNS-315 before with another HDD or is this a brand new unit?
While the DNS-315 is powered-up, press and hold the reset button on the rear of the unit for 8 seconds. This process will reset the DNS-315 to the factory default settings. If you still can't format the HDD after trying this approach, you can also try mounting the HDD in a PC to delete any existing partitions prior to mounting in the DNS-315 again. The DNS-315 formats HDDs using the Linux filesystem, so anything Windows does to the HDD will purge any formatting the DNS-315 may have done to the HDD.
You may also want to try mounting a different brand/model HDD as a test.