First see how much space is available in your device. For that, go to Settings --> Storage --> See the amount of space(how many GB) in "Available". If the size of app you are installing is more than what is shown in "Available", it won't install.
Try uninstalling a few apps from your device to make space for the new app. For that, go to Settings --> Apps --> Here you will see the list of apps installed on your device. Click/touch any app that you want to uninstall, then touch "Uninstall", then click OK.
You can also delete unwanted pictures from Photos to reclaim some more space.