Cap water supply line (kitchen)
The back of the cap packaging is generic and is meant more for other compression fittings.  It doesn't make sense for a cap.

That cap is the correct part you need, but Teflon tape is not needed for those threads.  In compression fittings, the threads do not seal anything.  The seal is made between "compression" of a ferrule that slides around the pipe being connected to.

With a cap, there is no ferrule, and the idea is that the seal will be metal to metal - the flat surface on the male threaded side of the tee, and the flat inside of the cap.  That cap will need to be tight to make a good seal.  If the face of the male threaded part is damaged or roughed up at all, a good seal might be hard to achieve.  In that case, you can cut a small washer to insert into that cap out of rubber or nylon or similar material.  That is usually not needed.