Normally a prepaid expense would not be deductible even for a cash based taxpayer unless you can justify the prepayment, such as getting a better price by locking it in now, or perhaps the trip would fill up and you could not go.
In TurboTax to deduct this you will need to enter the 2018 date paid and then in a note you can put for 2019 travel (in your records).
Just keep good records and details on why paid in 2018 versus 2019 for your records so you can show these if asked.
