Variables not defined

[ThiefOfTime]

This can not be the whole code since the imports are missing. The main problem is, that you have not defined the function plot_digits. There are two possibilities now. The first one would be that somewhere in the book they introduced this function so that you have to find that part and use it :)
The second would be that it is a function of some module like skilearn or so and you missed that import.
In both ways your compiler is missing this function and since the name of the function is not defined it throws that error :)