The error occurred because the default login mechanism tries to default login via the
pi user. You must have deleted or changed it in some way that the OS can't recognize it anymore. The solution would be to tell the OS to default login to a different user.
First of all. Login to a terminal from the error screen using
Ctrl+Alt+F1. Log in to the desired user.
Now when you have a functioning terminal, it's time to change the default user. The solution would be to create a task that automatically defaults to your preferred user:
sudo mkdir -pv /etc/systemd/system/[email protected]
sudo nano /etc/systemd/system/[email protected]/autologin.conf
In the file insert the following (note that the default username needs to be placed there as well)
ExecStart=-/sbin/agetty --autologin YOUR_DEFAULT_USER_HERE --noclear %I 38400 linux
Restart the raspberry and you should be good to go.
The error originally occurred because debian stopped using the inittab mechanism and started using
systemd. For additional information you can checkout a similar question