Error while doing tutorial

I was doing the Library tutorial to the letter, but when I reached this step:

It’s giving import error as you see in the image above, yet I am sure the Library Management exists otherwise I wouldn’t reach the tutorial that far.

Any idea what is that?

PS: I know the tutorial is for v13, but I got v12 in the virtualBox, there were few differences in codes like for example I am setting docType to numbers instead of using the frappe.model.docstatus which doesn’t seem to exist anymore, but other than that it should be the same, I guess.

I am using ERPNext: v12.29.0 (version-12)

Resolved when i rebooted the ubuntu. I dunno why.