1👍
I believe, the problem is with Object creation. I think the following is what you are looking for :
....
....
for i in l[1:]:
cat_1 = m.Category.objects.get_or_create(category_name = i[4])
cat_2 = m.Category.objects.get_or_create(category_name = i[11])
....
....
prod.category.add(cat_1)
prod.category.add(cat_2)
Source:stackexchange.com