Cannot open or update iPhoto
As you know, since the previous owners installed a version of the iLife pack you couldn't run on macOS Sierra, you've had to update the individual apps to their current versions. 

Where you've fallen down a little, is that while you could install current versions of GarageBand and iMovie, there is no current version of iPhoto as it was superseded by the launch of the Photos app. More specifically, iPhoto was discontinued in April 2015 and has not appeared on the Mac App Store since. 

Instead, what you need to do is use the Photos app. This should be on your MacBook Pro as it would've been installed as part of macOS Sierra. By default it should be on your Dock, but if it isn't you should be able to find it within your Applications folder.

If you have an old iPhoto library, Photos can convert it to a Photos photo library.

For more information, refer to Photos Help.