Problem
You can't add a task to a Point of Sale (POS) invoice.
Solution
Tasks, materials, and equipment can only be added to a POS invoice if their Pricebook category has been added to the POS location.
Go to the navigation bar and click Pricebook > Services.
Find the task you are having issues with and click Actions > View/Edit Service.
Make note the task's category in the Categories field.
Go to the Settings > Invoicing > POS Locations.
On the POS Locations screen, check the Pricebook Categories list on an active POS location to see if your task's category is on the list.
If it's not on the list, click Active > Edit.
On the Edit POS Location screen, add your task's category in the Pricebook Categories field.
When finished, click Save.
If you have more than one POS location, repeat steps 6-8 for each location.
Try adding your task on the POS invoice again.