Hi, I also have the same problem.
Tried this:
Hello!
I would try logging into your Google Account on a PC and then go to Android Device Manager. From there you can setup a new password and GPS/Locate your device. - My device cannot be located. Phone is right beside me and turned on.
Also you can try entering your main PW wrong a few times and the phone usually will prompt for your Google Account to unlock the device if you would rather do than (or don't have access to ADM). - there is no prompt for google account to unlock.
Also tried to lock my phone using the ADM by changing to a new password but it also does not work.
What should I do? Please help me. I have important messages in my phone. Thank you very much.