Hello @wenitwa,
I think you are probably right that you need the original state of the notebook, because the code that triggered the error should be in Cell #8. It being in Cell #7 is an indication that a previous code cell goes missing either because it was deleted or it was converted from a code cell to another type of cell. In fact, the original code cell #7 is an exercise that defines the variable model.
We cannot just revert part of a notebook to its original state, but we can get a fresh copy of the notebook. Here are the steps. Note that the steps will ask you to rename your current notebook before the system can give you a fresh one, but renaming it also means that both your current and the fresh ones will be kept.
Cheers,
Raymond