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.