Attaching a coupling to a copper pipe
I am not quite sure just what you are trying to do but from the picture you posted, the fitting on the top is a standard fitting for utilizing 1/4" OD plastic tubing and the bottom fitting is a standard 1/4" OD compression fitting. To use the top fitting with plastic tubing you cut the end of the tubing with a sharp object for a clean cut. Then, you slip the compression nut onto the tubing, then the 1/4" OD compression furrel. Next you push that small brass sleeve into the end of the tubing. Place the now adapted plastic tubing into that compression fitting (union) and tighten the compression nut. That small sleeve gives the plastic tubing strength to allow the compression nut to seal between the plastic and the nut. By the way, do not use the bulk type tubing, only use the tubing sold as ice maker tubing. That other stuff can break and cause water damage as I found out the hard way.