No, don't enter your 457(b) plan contributions under Deductions & Credits. Your 457(b) contributions are shown in Box 12 of your W-2 with code G, so you only need to enter this information on the W-2 screen.
Please see the FAQ below for instructions on how to enter the W-2 Box 12 information in TurboTax.
How do I fill out the Box 12 information from my W-2?
https://ttlc.intuit.com/replies/4785646
