For whatever reason the disk is improperly formatted, or something else is preventing communication with the Mac.
- Shut down the Mac.
- Disconnect the USB cable.
- Power on the Mac, log in, then launch Disk Utility: Disk Utility User Guide for Mac - Apple Support
- Select the Disk Utility's View menu > Show All Devices
- Connect the USB cable.
- If the drive has its own power supply, plug in its power cord.
- Select the device in Disk Utility and erase it. When erasing it, be sure to select the topmost device that corresponds to the Seagate drive — not a volume or container subordinate to that device. Click the Erase button.
- For Name: choose a name that is convenient for you. For example, "Backup" is ok.
- For Format: choose APFS
- For Scheme: choose GUID Partition Map
- Click Erase.
- Follow the rest of Apple's instructions for Time Machine: Back up your Mac with Time Machine - Apple Support
If something does not work as described above, please explain the step at which you're getting stuck.
If there is a problem with that hard disk drive or its USB connection, I suspect it may be Step 7. If the drive is broken or unusable, it might not even appear in Disk Utility. That's a show-stopper, but there might be some steps you can take to definitively rule out that possibility. Write back if you need help.
