Fulfill Checkbox greyed out / Not able to edit on Item Fulfillment

If you are not able to see the Fulfill checkbox on the line level of the Item Fulfillment but the you can see the line item on the Sales Order record check the following:

1. Navigate to the Sales Order the fulfillment is created from
2. Edit the Sales Order
3. Ensure that the Closed checkbox is not selected for the Line Items you are looking to fulfill
4. If there is any lines closed: uncheck the Closed checkbox
5. Save the Sales Order
6. Edit the Item Fulfillment

Note: You should be able to Fulfill the line item.