Can this be fixed on my debian 9.3 off-line system? When I run #, message says
WARNING: The following packages cannot be authenticated!
Authentication warning overridden.
And when I run #, message says
W: The repository 'file:/os/debian/9.3.0/amd64/DVD/extract/01 stretch Release' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
I previously asked this question for debian 8, I was told it can not be done. Now I an asking again for debian 9. APT-SECURE(8) says I need a Release file signing key. Ok, where can I get it?
For more details, see pdscript.com/d9316.txt.

What do you mean by off-line? The first command allows unauthenticated operation, but you give it nothing to do. Second command will update, but you are not allowing unauthenticated operation. At least this is how I understand it.

off-line means not connected to the internet, or any other system.
In this example, I omited package name from the first command because it happens to all packages.
In actual use, I do add a package name.

There are several solutions:
-Get Debian CD-ROMs
-Create a local mirror
-Download needed packages on another computer
-...

it's only a warning.
there is no problem.
i guess you could switch off ALL warnings in apt settings, but then are you sure you really want to do that?

Show us your sources?

Is this forum working? I posted, but the post did not stay. I said, answers are in pdscript.com/d9316.txt

ok, I guess it can not be done.