The unknown rune can't be the same as another rune in the expression. So here ? can't be 1, because 1 is otherwise present in the expression.

Yes. I also got the same problem: <2> expected but <1> found.

No, read the description again

If there are ?s in an expression, they represent a digit rune that the professor doesn't know (never an operator, and never a leading -). All of the ?s in an expression will represent the same digit (0-9), and it won't be one of the other given digits in the expression.

1 is already there. Read TEBESKASHI's post below, I already explained there this same test case.