This message appears on your phone when there is not enough available space on the internal storage for you to install new applications, so we need to free up some space!
First of all we can start with removing any cached data in your phone since this will become quite large after a while. To do that, go to Settings > Storage > Cached Data > tap on it > OK. This will delete the cached data for all the applications in your phone.
Next up is uninstalling applications that you aren't using. Go to Settings > Apps and check in the list if you have any applications installed that you do not use any more > tap on the application > Uninstall.
The preinstalled applications can't be uninstalled and here you will have the option to Uninstall updates instead. This will restore the application back to the factory version which is smaller in size. Do note that you will need to turn off automatic updates in the Google Play store, otherwise the applications will be updated again.
To turn that setting off, open up the Google Play store > 3 lines in the top right corner > Settings > Auto-update apps > Do not auto-update apps.