You could have deducted your property tax when you paid it, in 2017, even though tax for 2108 had not been billed yet.
You probably actually received a 'Notice of Value' in 2017 for 2018 property taxes, so check your files. These are usually sent out way before the actual Tax Bill comes out, letting you know the value of your property that the taxes will be based on. This is considered an 'Assessment'.
Open your saved 2017 return and add the amount of property tax you paid.
It if increases your refund, Amend your 2017 return.
Here's how to Amend:
https://ttlc.intuit.com/replies/4204352
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