the disk is locked OSX Sierra

Are you sure the SSD is locked? What do you find in Disk Utility? From what are you attempting to reinstall Sierra? Exactly how did you erase the SSD and try reinstalling macOS?

Clean Install of El Capitan or Later on a Clean Disk

1. Restart the computer. Immediately after the chime hold down the Command-Option-R keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the drive (usually, the out-dented entry) from the side list.
5. Click on the Erase tab in Disk Utility's main window. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to Mac OS Extended (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Reinstall OS X and click on the Continue button.

Use Command-Option-Rto install the version of OS X that was currently installed. Use Command-Option-Shift-R to install the original factory version when the computer was new.

You cannot downgrade without first erasing the drive. Then you need have a way to install Sierra. You would either need a copy of the Sierra full installer application or re-download the installer from the Purchases page in App Store or have made a bootable flash drive installer for Sierra. Or you can do the following:

Clean Install of El Capitan or Later on a Clean Disk

1. Restart the computer. Immediately after the chime hold down the Command-Option-SHIFT-R keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the drive (usually, the out-dented entry) from the side list.
5. Click on the Erase tab in Disk Utility's main window. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to Mac OS Extended (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Reinstall OS X and click on the Continue button.

(To install the version of OS X that was currently installed use Command-Option-R. To install the original factory version when the computer was new use Command-Option-Shift-R.)

The above will install the original OS X that came with your computer from the factory is you use the proper shortcut keys as provided in Step 1.

Also, read macOS Reversion- How to Downgrade from High Sierra and How to Downgrade macOS High Sierra.