This probably means that python doesn't know where PyQt5 is located. To check, go into the interactive terminal and type:
import sys
print sys.path
What you probably need to do is add the directory that contains the PyQt5 module to your PYTHONPATH environment variable. If you use bash, here's how:
~/.bashrcexport PYTHONPATH=/path/to/PyQt5/directory:$PYTHONPATH
where /path/to/PyQt5/directory is the path to the folder where the PyQt5 library is located.