Predicting which drug is most appropriate for which patient returns an accuracy of almost 0
I can't say exactly why you get null accuracy, but I have some comments that may help:

you are mixing continous data and categorical data. You might already be aware:

when standardizing your data: you are although standardizing your categorical data (sex, disease_type), just be sure that it makes sense (depending on the classifier you're using)
depending on the classifier, mixing continous and binary data may lead to undesirable results. This post explains it in the case of KNN



Otherwise I don't see anything suspicous after a first look. Could you share more data so we can reproduce your result ?