In the modern view, you cannot key in a transaction without selecting any item.
To key in services in the item maintenance:
Go to Maintenance, Item Maintenance, and Item File Maintenance.
Key in the services for your business.
Click More Info (2).
At the Item Type, click the drop-down button, and choose Service and Save.
Then go to Invoice, Create Invoice, click '+' icon to add an item.
Choose Service at the item type and proceed with the Invoice.
Note: Ensure the Service No. (Service Maintenance) is different from the Item No. (Item Maintenance). If both share the same name, the system will deduct the Item by default.