If you are having issues logging into your COD or Warzone account and see the registration Error 0. Follow these 5 steps in this video and it should fix the problem.
A Registration error will occur if you attempt to register for an account in game using an email address that is already tied to an existing Activision / Call of Duty account.
So, to get around this issue you need to follow the following steps.
Close you game and do a hard reset on your console or restart your PC. (hold the power button for 10 seconds on Xbox)
Try and login again using your Activision email and password. If this doesn’t work on your web browser go and login via your network (PSN / XBOX / STEAM) or email and password.
Make sure the account you want to use (XBOX / PSN / STEAM) is linked up to your Activision account.
Once you have linked your account try and login again after restarting your console or PC.
If this still doesn’t work, try resetting your password using the link below. Follow the on-screen instructions and once this has been done try and login again with new password.
Hope this has helped you and as always leave any questions below in the comments.