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. 

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), pages A-13 to A-17.
 
To enter sales tax rates in the TaxAct program:

1. From within your TaxAct return (Online or Desktop), click Federal. On smaller devices, click the menu icon in the upper left-hand corner, then select Federal
2. Click Itemized or Standard Deductions to expand the category and then click Taxes paid
3. Continue to the screen titled Do you want to enter state and local sales tax information? and click Yes
4. Click Table Amount and enter the appropriate information
5. Click Continue until you come to the screen titled What is your local sales tax rate? and enter the appropriate tax rates