Problem
You can't add charges to an invoice because you're getting an error message. The job was completed by multiple technicians with assigned splits to the job but the technicians are now inactive.
Solution
Splits on jobs decide if you can add non-zero dollar items to an invoice. If an inactive technician has a 100% split, you won't be able to add items on the invoices. In this case, you will need to adjust the technician split on the invoice and assign 100% to an active technician.
Adjust technician split on invoice
Search for and go to the invoice.
On the invoice record, click Adjust splits.
On the Update Splits screen, find an active managed technician and set Sold Hours (%) to 100.
If there is no active managed technician available, click Add another.
On the new row that appears, select an active managed technician from the dropdown and set the Sold Hours (%) to 100.
When finished, click Save.
You should be able to add charges to the invoices.