If you are claimed as a dependent on another return, you cannot use the Optional State and Certain Local Sales Tax Tables for calculating an itemized deduction for General Sales Taxes on Line 5 of Schedule A (Form 1040) Itemized Deductions.
You would be eligible to claim the actual state and local general sales tax expenses, but would need to keep your actual receipts showing the general sales taxes paid.
Sales Tax Tables can be found in the Instructions for Schedule A (Form 1040), on pages A-13 to A-16.
To enter sales tax rates in the TaxAct program:
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.