Sometimes, when registering the usage of a product during a Task, the item in question will be missing from the dropdown menu. This can occur during certain tasks where a default unit is set.
For example, if your default planting unit is set to pieces, you cannot register planting of a product whose unit is weight.
First, check the planting unit in your Crop Production settings. If your planting unit does not match the planting item's unit of measure, you may need to re-add this item to your inventory with the correct unit, or process the item into the correct unit (via Options → "Register Processing").