You must reformat the drive to use HFS+ because Sierra cannot be installed on an APFS formatted disk. Why do you wish to downgrade to Sierra? Fortunately, you can accomplish what you want using Network Restore:
Internet/Network Recovery of El Capitan or Later on a Clean Disk
If possible back up your files before proceeding.
- Restart the computer. Immediately after the chime hold down the (Command-Option-Shift-R) keys until a globe appears.
- The Utility Menu will appear in from 5-20 minutes. Be patient.
- Select Disk Utility and click on the Continue button.
- When Disk Utility loads select the drive (usually, the out-dented entry) from the side list.
- Click on the Erase tab in Disk Utility's main window. A panel will drop down.
- Set the partition scheme to GUID.
- Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
- Click on the Apply button, then click on the Done button when it activates.
- Quit Disk Utility and return to the Utility Menu.
- Select Reinstall OS X and click on the Continue button.
This will install the original OS version pre-installed when the computer was new (10.6.6 or .7). From there you can download the Sierra installer and upgrade.
