I can verify PixelCake's issue, and elaborate on Mario's reply. You don't specifically need the older version to be installed - literally any other version present on the system will allow you to upgrade.
With only one Unity version installed, you will get no dropdown under the Unity Version column, and the version number appears as simple text. Clicking the text does nothing. Clicking the project name fails to open the project, and displays the error as PixelCake reported.
To reproduce this effect, install Unity 2019.2.6f1 from the download page, and create and save an empty project with that version. Uninstall all Unity versions, then install any higher version (as of this writing, 2019.2.8f1 is the highest). Make sure this is the only version installed. Then, try to open your project. The "Unity Version" column will be text, not a dropdown, showing the only installed Unity version. Opening the project through Unity Hub will be impossible, giving the error reported in the original post:
However! Now install any other Unity version. In this example (just to illustrate the point) I installed a very old Unity version, 2017.1.5f1. With two+ versions available, the dropdown is now present, and the project can be opened in 2019.2.8f1:
Note that the little orange caution sign is gone. You can now select the higher version, upgrade the project, then delete the dummy install that forced the dropdown to appear.
Assertion: I should be able to upgrade my project to a newer version of Unity, even if that's the only version installed.