My husband and I are filing a married filing joint return. We are both 70 years old and have one dependent, age 18. Turbo Tax is showing no personal exemptions. Why?

There are no longer personal exemptions under the new tax laws.  The personal exemptions were replaced with higher standard deductions and the child tax credit or Other Dependent Credit.  If you are claiming the 18-year-old and have a tax liability you should see a $500 credit.  

If you do not have a tax liability, you will not get this credit.  

Turbo Tax says I owe $10K+ more than I've already had withheld.  Where would I see the $500 credit?

line 12 a of your 1040