Compression fitting with copper nut and plastic pipe keeps leaking
With plastic pipe compression fittings usually come with an insert made of metal or hard plastic.  The compression nut and ferrule (generally plastic for plastic pipe) is placed over the pipe, then the insert is pushed into the end of the pipe.  When the joint is tightened the insert prevents the plastic pipe from collapsing as the ferrule bites in.  Without the insert it is very difficult to make a compression connection in plastic watertight.

See the second question in the FAQ on this page: https://www.plumbingsupply.com/ed-compression.html