When you go to System Preferences > Users & Groups > Login Options on your Mac, the option for "Automatic login" may be grayed out and cannot be enabled.
The "Users & Groups" settings may be locked for security purposes or encryption may be enabled on your Mac.
If there is a lock icon in the bottom left-hand corner of the "Users & Groups" window and the icon is locked, click it to unlock it.
Type the username and password of an account on your Mac with administrative privileges. Then, click Unlock.
If the lock was already unlocked and the "Automatic login" option is still grayed out, then you likely have encryption enabled. If so, there is no known resolution at this time. The behavior is expected.