Problem
I can't assign a new technician to new appointments on the existing work orders, as I am getting Action is cancelled, at least one managed technician must be assigned to this appointment, since the work order total >$0 error message.
Solution
Set technician splits
This error message occurs when the technician assigned to the work order is unmanaged or deactivated. To resolve the issue:
Open the invoice for the work order.
In the side menu, click Adjust Splits.
Change the splits so that the managed and active technicians have 100% of the splits, and the non-managed or deactivated technician has 0%.

When finished, click Save.
For more, see Set technician splits.