tried to clean up my mac to sell but now my OS X hard rive is locked and I can't reinstall OS

[iMore Question]

I have a Late 2015 iMac, base model (cheapest)
Tried to get all my info off the computer before sale and return it to a like new state.Recently uprgraded the mac to OS sierraFollowed instructions form this website. However, now I can't partition the hard drive nor can I unlock it to finish the install. Need help quickly or this sale is in the toilet! Help!!!