Model Assessment block, in this case, evaluates whether the renamed signals are correct or not.
You're getting that error because you've named the signals incorrectly.
See this for instance. Here, the error is due to the fact that I made a typo (x_kminus11) instead of x_kminus1.
If you want further help to resolve this, please make use of Hint and See Solution in the left hand side pane.
