This error results because for whatever reason, Mechanize is seeing multiple username/password fields. The index parameter must be used to select the correct one. Comment out the following in the script:
#br.form.new_control('text', 'login[username]', {'value': username}) # Had to manually add username control.
#br.form.fixup()
#br['login[username]'] = username
#br['login[password]'] = password
And replace it with this:
userone = br.find_control(name="login[username]", nr=0)
userone.value = username
pwone = br.find_control(name="login[password]", nr=0)
pwone.value = password
This should fix the issue and allow Mechanize to log in. Note that more modification to the script will be required per the Ippsec video and official write-up.