The system isn't designed to let you reset the password easily, because that would also allow a thief to bypass it.
Did you have a password set before it was stolen? I believe it doesn't actually change the password if you have (whatever it says), so in that case your old password should work. Unfortunately that almost certainly means you can't use Android Device Manager to reset the password now.
Older phones will prompt you for your Google account credentials if you enter the wrong password often enough. Some will however do a factory reset in this case, and this won't work with more recent phones.
Of course a factory reset should remove a phone password, so as a last resort you can try that. Make sure you know the username/password of the Google account on the phone though, because all phones released in the last few years will have Factory Reset Protection, and if you reset it without knowing those credentials you can be really locked-out.