Hi,
Do you have NCL installed? If not, take a look here: http://www.ncl.ucar.edu/
Make sure to download and install the pre-built binaries, and do not try to install from scratch.
If you have it installed, it may be that you just need to point to the binary in your path, and you'll also need to set the NCARG_ROOT environment variable. This presentation about NCL may also have some useful information for getting things set up:
https://www2.mmm.ucar.edu/wrf/users/tutorial/202001/jaye_ncl.pdf