I withdrew contribution only money from my Roth IRA in 2017. The 1099-R shows "Taxable amount not determined." IRS sent proposed changes to my return. How to respond?

what were the code letters in box 7 of the 1099.  what year was contribution 

Did you enter the Form 1099-R into TurboTax?

Did TurboTax produce Form 8606 and, if so, did the form show a zero taxable amount?

A distribution from a Roth IRA reported with code J (or with code T and it has been less than 5 years since the beginning of the year for which you first made a contribution) in box 7 of the Form 1099-R is required to have been reported on Form 8606 Part III unless the Form 8606.  In this case, if your tax return did not include Form 8606 you need to prepare Form 8606 Part III and provide it to the IRS.  The IRS usually asks for this Form if you failed to report a distribution