I disagree with the statement that sending criminals to prison is not effective, but I agree that education and job training could be one of the effective approaches to rehabilitate the criminal mindsets by the standards of the society.
I believe sending lawbreakers to jail should not be a concerning point. Point of concern should be that if someone is arrested for robbery and is sharing their jail time with a murderer or someone who has committed a much serious crime, they have more possibility of joining the same personality trait of a murderer in future because it is a strong part of human psychology that we adapt to our surrounding. Setting punishments by sending criminals not only sends a message to the whole society about the consequences of the crime but also separates the individual from the society and gives them a chance to re-think their actions and their consequences while protecting society from further damage.
The prison system should not only be for punishing these individuals but also needs to work as a rehabilitation system for them. For example if someone robbed a bank/shop the most likely reason behind it would be unemployment or the lack of skills to earn money for themselves that led to a thieving mindset, by spending jail time and work on their learning skills and get psychiatric care these individuals can become a productive part of the society.
On the other hand individuals with more heinous crimes like murderers or serial killers should be kept in these facilitated for as long as it is clear that they have completely rehabilitated to the standards of the society and are no longer danger to the people around them is crucial.