"...I can allow him to live with us but I tell you, I will not going to teach him, I only gave my word to be a teacher for you. If you want him to be with us, You have to teach him all right and complete knowledge.
You are not allowed to make mistakes, You already chose to take over the holy and sacred position of a Guru. If you are not able to reach him nicely, Be ready to face my wrath."
Durvasa for a second fell into thinking, It was not that big of a condition like his previous ones but because Karna already took a student is a big problem.
It's forbidden to take a student when you yourself are not knowledgeable because it will be disrespectful to the pure bond between a guru and his student.
The Guru is considered as a second father for the student And because of that it's taken very seriously if a Guru is tricking his student, The punishment is worse than death.