Apple HDD SATA not erasing or mounting

Hi! I am attempting to restore an old Mac Mini (I believe 2012, but could be older) that is currently running El Capitan (OS X, 10.11.6). This is a work Mac Mini, and I am hoping to be able to use it at my station after a coworker left. When I first received the Mac, it gave me a prohibitory symbol (circle with a slash) and after checking discussions, I was able to successfully reinstall El Capitan on the 128GB SSD and startup the device and create a User account. Importantly, the Mac also comes equipped with a 1TB HDD that I would like to be able to access. After reinstalling the OS X and going back to disk utility, I notice that the Apple HDD, is grayed out (see photo).

I performed a "First Aid" to which it found the partition is running properly. However, I am unable to erase and mount the disk. It keeps giving the following error when I try (photo below):

newfs_hfs: WriteBuffer: pwrite(3, 0x7fad19036000, 512, 1024): input/output error

newfs_hfs: write (sector 2): input/output error

Could not mount disk1s2 with name (null) after erase

File system formatter failed

Operation failed...

Because of this, the Mac says I only have 128GB of storage and I would like to access the rest.

Any advice on how I can erase and remount the HDD? Any help would be appreciated. I don't mind erasing the data or Mac HD and starting over again since I want it to be essentially as empty and brand new as possible.

Thanks in advance.