This issue occurred because the billing license was not detected in the sub-PC UBS system. To fix this issue, link the sub-pc license to the server:
Log in to the UBS system on Sub-PC.
Go to Admin, Administrator Settings.
Go to System Settings.
Check the Server IP Address under the Inventory & Billing tab, make sure it is the same as the Server IP Address under Accounting.
If different, please change it by following what is stated in the Accounting tab.
Once changed, proceed to save it.
After finishing, open the Billing module - it should solve the issue.