Is it religiously obligatory to inform the manager of insufficient qualification to train a specific program, and is training trainees with the manager's knowledge of this considered deception and injustice to them, and does sin accrue to the trainer in this case after informing his manager?

You must inform the manager of your incompetence for the required training, as this is a matter of necessary advice and trustworthiness.

If the manager insists on assigning you the training after you have informed him of your unsuitability, then it is not permissible to obey him. If you obey him while you are not competent, then sin falls upon both of you due to the deception of the trainees.

However, if the manager knows of your competence for the training, and you merely think otherwise out of humility, then it is permissible for him to assign you, and there is no sin upon either of you.