Hi there, @LX2.
Welcome to the Community. Normally, the field to implement a county sales tax is greyed out if you haven't activated the sales tax in your Company Preferences. Let me help you set up your sales tax so you're able to add an item for each county, district, parish, etc. Here's how:
1. Go to the Edit menu, then select Preferences.
2. Choose Sales Tax, then click the Company Preferences tab.
3. Select Yes to turn on sales tax.
4. Click the Add sales tax item to add it for each county, district, city, etc. where you collect sales tax. Make sure you check the tax rates and requirements with your tax agency, then follow steps below:
To assign sales tax codes, read through step 5 in this help article: Set Up Sales Tax in QuickBooks Desktop. Click each link to help you complete other sales tax-related tasks.
Feel free to post again if you need help with anything else about QuickBooks. We're glad to be of your service. Have pleasant day ahead!