Warning when plotting confusion matrix with all sample of one class
If you insist on giving no positive examples it will have the same problem. As the error message states, the tool wants to compute the recall. The recall metric is given by:
$$recall = \frac{tp}{tp+fn}$$
$tp$ (true positive) is equal with the number of positive cases predicted as positive, which is $0$ in your case.
$fn$ (false negatives) is the number of positive cases predicted as negative, which again is $0$.
What you ask is to compute $\frac{0}{0}$ which is undefined. You can add positive cases also or you should study the tool (I'm not too familiar with Python stack)and perhaps you find an option to not compute recall metrics for you.