Should Schedule A box 5a for sales tax be checked if you itemize and have no state income tax? I could not find a way to add it. How do I do that?

You can enter the sales tax you paid in  2019 by going to Federal>Deductions and Credits>Estimates and Other Taxes Paid> Sales Tax. 

Sales tax is an itemized deduction.  If your state has a state income tax, you have to choose between using the deduction for your state tax paid OR the sales tax deduction, whichever is higher.  The TT process will suggest one or the other as you prepare your return.

“Major purchases” that you can enter for the sales tax deduction include:

Motor Vehicles (cars, trucks, motor homes, RV’s, sport utility vehicles and off-road vehicles

Aircraft or boats

Mobile homes

Manufactured housing

Building materials for major home improvements

You cannot deduct: furniture, jewelry, home electronics such as TV’s or computers

https://ttlc.intuit.com/questions/1901222-which-deduction-should-i-choose-sales-tax-or-income-tax

https://ttlc.intuit.com/questions/2566624-how-much-sales-tax-did-i-pay-last-year

https://ttlc.intuit.com/questions/3148443-how-do-you-define-major-purchase-or-major-item-for-the-sales-tax-deduction

https://ttlc.intuit.com/questions/1900791-how-do-i-find-my-local-sales-tax

https://ttlc.intuit.com/questions/2566624-how-much-sales-tax-did-i-pay-last-year

If you have not entered state sales taxes, type sales taxes paid in the search box and click on jump to sales taxes paid. You can either enter actual taxes paid or use the Easy Guide to enter the taxes from IRS tables based on your income at the local and state sales tax rate. If you paid sales tax on certain major purchases such as a car, boat, building materials or other specific major purchases, you can add that sales tax to the table amount.