I am trying to install a package called \"simpleguitk\" via pip. (On Ubuntu 16.04 with Python 3.5)
After running
sudo -H pip3 install simpleguitk
The issue could be that the version of python you used to install the module does not match the version python you are trying to import from.
Find out whether the module in the python version you wanted
you can try using the command: pip3 freeze to get the list of packages installed for version of python(In your case, it is python3.5).
Before that, check different versions of python installed in your machine. You can use the command locate /python | grep /bin
if you have python2.7 and python3.5, then you should use the corresponding pip/pip3 to install the modules.
Open the corresponding python shell (python3) and try to import again