The TAPI driver allows only deleting the last TAPI line (the TAPI line with the highest index).
The reason is that the TAPI applications will open the TAPI lines using the index. If a line between would be deleted, the line indices would move and then the TAPI applications may open a wrong line.
The column to the Line-Idx is initially invisible and only appears when you click the [Delete] button.
Sort the list if necessary by the Line-Idx to see which line has the highest index.
If you no longer need a line that is in between, this is no problem for now. You can ignore this line at first and possibly reconfigure it if you need a new line.
The license management counts only the number of actually opened Lines.