You can deduct your 2018 property tax on your 2017 tax return only if these property taxes have been assessed and paid in 2017.
If you made a payment in 2017 for 2018 property taxes in advance of them being assessed, then that payment cannot be deducted 1n 2017. Please read this IRS document for more information:
https://www.irs.gov/newsroom/irs-advisory-prepaid-real-property-taxes-may-be-deductible-in-2017-if-a...