Gordon,
If you just installed AppStudio for Desktop, I would assume that you are running AppStudio Desktop version 3.1.134, correct?
And can you tell us a little bit more about the app you are trying to run? Because it looks like it is using an invalid (maybe older version) of the import statement to load ArcGIS Runtime library.
In AppStudio Desktop 3.1.134, it comes with ArcGIS Runtime SDK for Qt version 100.3, and the import statement should be as follow,
import Esri.ArcGISRuntime 100.3
You can try fixing the import statement to load ArcGIS Runtime library in the files and line number stated in the error message.
Thank you,
Erwin