Q: What does unknown0023121250f5 mean and how can I fix it?

When Terminal is open, do a command shift 4 and draw a box around the Terminal and then it appears on the Desktop

Use the camera thingy in the post here to upload it.

When you open System Preferences > Sharing, what appears in the Computer Name box at the top?

sudo scutil --set HostName whatever-you-want

The hostname is being set to the result of a reverse lookup of your IP address on a local DNS server, most likely your gateway.

That's your user name.

What you see in the command prompt is the "short" user name, although this one isn't especially short. What you see in the login screen is the "full" name.

I don't understand the question. You're logged in as perrywilliams.