If you can see what appears to be line items extracted on an invoice but a description is not showing/able to be added, this is most likely a Tax Summary rather than line items.
If you wish to be able to add descriptions to these lines, you will need to first remove the lines in this section that have already been extracted. You can do so by clicking the red 'X's to the right of each line. Once these have been removed, the option to Add Line Items will become available.
You can then choose the option to Add Line Items and manually re-add these lines and add descriptions to these lines.