You can not enter a Negative RRSP Deduction in TurboTax. If you do have a negative number here, CRA will make the adjustments once you file your return with "earned income" and open up more room.
If you have a negative number here and have made a contribution when you are considered to have Zero room, you will see an effect from CRA on the Excess Contribution Amount of $2,000 that is allowed. CRA will also adjust your deduction limit when you file to agree with their records showing the negative amount.
From this article at TaxTips.ca: RRSP Contribution Limits A taxpayer can contribute up to the amount of their deduction limit, plus an excess contribution as long as the total excess contribution never exceeds $2,000. However, the allowed excess will be less than $2,000 when the deduction limit is negative due to a PSPA amount. Also, only taxpayers who are 19 or older in the taxation year qualify to have an excess amount. Any excess contribution over $2,000 may be subject to a 1% per month tax.