I modified the tax rate to add CESS percentage but it is not reflecting in the sale voucher. What am I doing wrong?

If you have already defined Cess within the tax category that is used in the item master, but it is still not applying in the sale voucher in BUSY, it depends on the type of sale you are using. Below are the steps to troubleshoot:
 Step 1: If you are using the sale type as Local/Central item-wise, then when you enter an item in the item body and click on the amount column, the cess will automatically apply with other taxes. You can check the same bifurcation in the tax summary.
 Step 2: If you are using the sale type as Local/Central Multi-rate, you have to apply cess with tax in the bill sundry by clicking on the apply tax option. Additionally, you have to ensure that the bill sundry of cess is created. Although when you enable the cess, it automatically shows in the bill sundry list, if it's not created, you have to create it by following the steps given below:
 - Go to the Administration tab.
 - Click on Master.
 - Click on Bill Sundry and then add and create the bill sundry of cess from there.
 Step 3: If you are using any specific tax rate sale type, you have to mention the cess percentage within that sale type. To modify the sale type, keep the cursor on that sale type, then press ALT+M. It will open in modification mode, and you can define the cess rate from there.