Where is the option to require purchase orders to be approved located?

Approval feature in BUSY enterprise, please follow below steps 
Step 1. Click on Administration.
Step 2. Go to Configuration.
Step 3. Select Features Options.
Step 4. Navigate to the Enterprise Features tab.
Step 5. Enable the Enable Voucher Master Approval option.
Step 6. Click on the Configuration button.
Step 7. In the User Who Can Approve section, select the sub-users who should have approval rights.
Step 8 : In this windoe ypu can find conditional approval . you can set the rules for the approval . Save the changes
Selective approval in BUSY allows you to specify criteria for which approval is required for certain transactions.
Step 1 :Pressing the F4 key triggers an action in the software, in this case, to approve or reject the selected vouchers or masters.
Step 2 :After pressing F4 a message box appears with two options: Approve and Reject. You need to choose one of these options based on whether you want to approve or reject the selected items.
Step 3 :Once you select Approve or Reject, another window appears where you may be prompted to specify 'Y' to confirm the action. You can also provide remarks or comments explaining the reason for the approval or rejection.
Step 4 :After specifying 'Y' and providing remarks, the software will proceed to approve or reject the selected vouchers or masters based on your input.