Hi brittg13,
The MG3120 is not officially supported on 10.14 so I can't guarantee that the drivers and software will work properly. For MP Navigator to work, the scanner driver will need to be installed as well.
You can click the link HERE to get to the download page for the MG3120. In the list of drivers that appears, you can click the operating system drop down box to select "macOS High Sierra". Once the OS has been selected, you can click select and then download next to the MG3100 series Scanner Driver. Once the download is complete, you can double click on the download to start installing the driver. Once the install is complete, you can try reopening MP Navigator.
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