As part of the MyNMIT create an account process, after you confirm your email you are taken to a page to set a new password.
This page displays the password criteria that your phrase must match in order to be accepted. All of the criteria must be satisfied, especially the last point for data breaches.
Once all the grey icons have turned green the final button Set password will turn blue and you can successfully submit. Only then will you have set the password and will have completed the process.
You will then be taken to a success page and can sign in to MyNMIT using your email address and newly set password.