Unable to open recovery disc. Am told it is "locked", how do I unlock it?

Began upgrade from Sierra to High Sierra today and the upgrade stopped to to an error. Cant remember what the error was though. Anyhow, So I restarted the computer holding option. Had the choice of selecting either "Basic OSX" or my recovery disc of the Sierra backup. I selected the Sierra backup, click next and was told that it was "locked". I cant figure out how to unlock it so I can re install it.

Help would be much appreciated as I have commited the cardinal sin of not having an external backup.

Thanks