I need authorization from Google before downloading Messenger but it wants me to sign into my Google account. I am signed in everywhere I need to authorize to download Messenger. It says sign into your Google account, but I'm already signed in everywhere. I don't understand what I'm not doing right.
You need to relogin as it may be a play services error. just remove your google account/accounts, reboot your phone, relogin to your account/accounts. if it still fails then look for google play services in 


  settings > applications > google play services 


and do a clear data. after restarting, relogin your account/s it should work fine.