At Basic Data tab, click on the right pointing arrow at the bottom of the screen to display additional columns to the right, including the billing element (Bill) field, as shown below.
Note: If the Bill field is checked, then billing element is already enabled.
Click on the Bill field to enable billing element, as shown below.
Note: You can disable billing element (by unclicking the Bill field) only if no revenue has been posted to the WBS.
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at the bottom of the screen to display additional columns to the right, including the billing element (Bill) field, as shown below. 