Opening an existing project file in ArcGIS Pro failed and returned the following error:
Error: Opening the selected ArcGIS project failed. This project was saved using a newer version of ArcGIS Pro:<version number>.
The error message is returned when using ArcGIS Pro version 1.x to open a project file created in ArcGIS Pro version 2.x. This is an expected behavior. Projects (.aprx), project packages (.ppkx), and project templates (.aptx) saved with any version of ArcGIS Pro 1.x can be opened and used by any version of ArcGIS Pro. However, projects saved using ArcGIS Pro 2.x cannot be opened by ArcGIS Pro 1.x.
Upgrade ArcGIS Pro to the latest version. Refer to ArcGIS Pro: Update ArcGIS Pro to do so.