reformat hard drive and i reinstall mac os sierra it says this disk is locked

Install El Capitan or Later from Scratch

If possible back up your files.

1. Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:
2. Select Disk Utility from the Utility Menu and click on Continue button.
3. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
5. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Install OS X and click on the Continue button.

This will install the version of OS X you had installed.

If the disk is still reported as locked then it has failed and should be replaced.

The hard drive may have failed as Kappy posted.

If you live near an Apple Store, make a Genius Bar appointment to have the computer tested. Supposedly there is no charge for testing. Use 2nd link if not near an Apple Store or aren’t in the US. Hardware Repair - Keeping Confidential Data Safe
Take the original purchase receipt with you.

Genius Bar Reservation US

Authorized Service Provider