I am currently in an MPI position and with no credentials. Can the commander write a memo for me so I can do MPI duties?

You're stuck till you have the school. There is still plenty you can do. There is still tons of leg work that needs to be done on investigations. You need to know how to take statements and collect/log evidence without thinking about it before hitting school.

See if you can get the text books from somebody that has already been and read them. It is not a super hard course, but there is tons of information getting thrown at you. The more you got down before you go the better.

I know this has been answered several times over already, but AR 190-30 states installation provost marshals will ensure candidates have completed the Military Police Investigator's Course at USAMPS prior to performing investigative duties AND issue credentials only after completion of the course.

SFC (Join to see)
You won't be able to obtain creds until you finish MPI school. You can assist with interview and investigations but you are not allowed to run your own cases. I had MPI school en route to Germany last year and it took a couple months until I was able to get my creds so I was unable to run my own investigation. I hope this helps. If you have any questions, please feel free to contact me as I am currently an Investigator in Grafenwoehr, Germany. I'll help in any way possible.