Why can't I enter my password in the Mac mini Terminal?

If Terminal is asking for a password, then it is likely not going to show the password as you type it. The command line will remain blank.

Just type the password and then press the Return/Enter key.

Again, you will not see the password as you type it, but it is being processed.

[ Redacted /viking]

As I posted above, in this circumstance Terminal will not show you the password as you type it.

You must simply type your password in the blind and then press the return key.