Did you receive the per diem from your employer as a W2 employee? Because in that instance it is incumbent on your employer to handle the proper reporting, not you. If the per diem is issued to you to handle expenses related to your W2 employment then it is assumed that your employer has properly documented those expense instances as well as the amount provided to you and that none of that reached the level of compensation or it would have been reported to you on your W2.
I did not work enough events or make a certain amount of money to receive a 1099 form. There was no W2 as it was basically contractor work through a company.
Then you are self-employed. The per diem is the income that you were paid. You will go to the self-employed section of TurboTax and enter the amount that you received (even though you didn't get a 1099) as the total you received in cash. Then you will enter the expenses that you had - if you paid for food or equipment or travel with your money in order to perform the work it is deductible. Whatever is left over you will pay taxes on.