This error occurs when the tax rate applicable to the service category is not set.
To resolve the exception Unable to determine tax rate
1. Go to Gateway of Tally > Display > Statutory Reports > Service Tax Reports.
2. Click the category Unable to determine tax rate.
3. Drill down to the service category for which the rate is not set.
4. Enter the Service Tax Rate as shown below:
5. Press Ctrl+A to accept.