If Box 20 on your W-2 is blank and your tax software is asking for information, here are some steps to help:
Box 20 lists the locality name for local taxes withheld in Box 19. If Box 19 has an amount, Box 20 should list a city, county, or local tax jurisdiction
If local taxes were deducted but Box 20 is empty, check with your employer for the correct locality name
For more detailed information, you can refer to the IRS explanation of Form W-2 here