I removed Macintosh HD from backup disk how do I set it back?

I used 2 backup disks , Macintosh HD and Seagate . My macintosh was unable to perform a backup due to lack of space while my Seagate is unable to perform a backup until the first backup is complete. And I happened to remove Macintosh HD from the back up disk and I couldn't set it back because the name Macintosh HD is no longer under the "Available Disk" besides Seagate

How do I retrieve it back?