dwfaust is correct - When you are asked to enter a password in the terminal, no characters are displayed as you type in the password.
There can be some uses for a blank password, so when you see a password request, all you need to do is press enter.
However, sometimes the blank password is not a good idea, because (as you have discovered), you may find that a blank password is not suitable when a real password (with actual characters) is needed, and a blank cannot work.
You will find that some parts of the system will not let you use a no-character (blank) password.
Think of it this way - a blank password (no characters) is NOT the same as no password. Even the blank means that you still have a password, it just has no characters. … a-a-and, you can't enter a password that has no characters.
So, in certain situations (and with some applications) a blank password results in an error - and the only way to correct the error is to add a password for your admin user account.