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The project has already been invoiced, and the payables have been automatically locked.
Solution
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Unlock the payables.
Go to the Invoicing terms tab in the Finance card.
In the menu on the left, select the task in question.
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Cause 1: You are not in edit mode
Solution
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Open the payables in edit mode.
Click the Edit button at the bottom of the page.
Cause 2: The project has already been invoiced
Solution
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Unlock the payables.
Go to the Finance tab for the job in question.
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