Enter "Other - not classified" or "Other – not on above list". Your entry here will not affect your ability to e-file or your tax calculation.
Employers can put just about anything in box 14; it's a catch-all for items that don't have their own dedicated box on the W-2.
If you don't see a category in the TurboTax list that matches your W-2 entry (MAND 403B), don’t worry. We'll figure out if it impacts your return or not. If we need more information, we'll ask you.