In this example (just to illustrate the point) I installed a very old Unity version, 2017.1.5f1. However! Now install any other Unity version.

Opening the project through Unity Hub will be impossible, giving the error reported in the original post: The 'Unity Version' column will be text, not a dropdown, showing the only installed Unity version. Make sure this is the only version installed. Uninstall all Unity versions, then install any higher version (as of this writing, 2019.2.8f1 is the highest). To reproduce this effect, install Unity 2019.2.6f1 from the download page, and create and save an empty project with that version. Clicking the project name fails to open the project, and displays the error as PixelCake reported. With only one Unity version installed, you will get no dropdown under the Unity Version column, and the version number appears as simple text. You don't specifically need the older version to be installed - literally any other version present on the system will allow you to upgrade. Once the solution is loaded, Rider and the Unity Editor become connected.I can verify PixelCake's issue, and elaborate on Mario's reply. Rider will start automatically and open the solution related to this Unity project. While still in the Unity Editor, right-click anywhere in the Project view and select Open C# Project In the External Script Editor, select a JetBrains Rider installation. Select Edit | Preferences in the Unity Editor and open the External Tools page. Open an existing Unity project in the Unity Editor.

Start using Rider with your Unity project The plugin comes bundled with Rider, so manual installation is not required. Support for Unity is based on the open-source Unity Support plugin. JetBrains Rider provides support for Unity API versions 5.0 - 5.6, 2017.1 and later. JetBrains Rider provides powerful support for Unity including: Unity-specific quick fixes, context actions and inspections, code completion and syntax highlighting in shader files, debugging Unity scripts, running and debugging Unity tests, profiling Unity games, refreshing assets, and much more.

Unity is a game-development environment that is used to create 2D and 3D games and deploy them on multiple platforms.

Unity editor install