Authentication Required?

I have a home network with three Win10 computers on it. One of the three
has several external disk drives attached. All the computers can see each
other and all can access the external drives. Recently, I bought an Amazon
Firestick and attached it to a television in front of my treadmill. One of
the applications on the Firestick needs to see the external drives on the
network, but when I try to access the computer the drives are attached to,
I get a message that says that computer requires authentication and it
wants a username and password. I cannot for the life of me remember ever
setting up anything on the network to require authentication, and no
combination of usernames and passwords gets access. Can anyone help figure
out what's going on here and how I can log on to the Win10 computer from
the Firestick? Thanks...