Why, in the concluding paragraph, does Plato say it is impossible for professors of education to put knowledge in the soul which was not there before?

This is because the ability of learning has to already be in the soul

Explanation:

Plato said it is impossible for professors of education to put knowledge into the soul which was not there before because the ability of learning has to already be in the soul. One who does not already have this, cannot be taught.

Plato treats the subject of education in The Republic as an integral and vital part of a wider subject of the well-being of human society. The ultimate aim ofeducation is to help people know the Idea of the Good, which is to be virtuous.