The standard deduction gets input into your return when you first enter some income (like a W-2 or a 1099), therefore the standard deduction was in place before you starting entering in itemized deductions. Once you completed entering the itemized deductions, TurboTax let you know that the total itemized did not exceed the standard, therefore it would have left the standard deduction in place (so you would see no change in the amount due or refund).
To double check that your return is taking the standard deduction, you can preview your 1040 form before filing and see what amount is reported on line 8. See the steps below to do so in TurboTax online:
https://ttlc.intuit.com/replies/3302322
Below are the standard deduction amounts for 2018, which varies by filing status:
https://ttlc.intuit.com/replies/4208574