Thanks to the ZED Mini's inside-out tracking, you don't need to plug in the Oculus Rift's sensors to get full positional tracking on the headset. However, opening the Oculus Platform without any sensors plugged in (which normally happens when you run a ZED Mini app compatible with the Rift) will cause a window to appear inside the headset. You cannot use the keyboard to bypass this window.
We can't find your Sensor. Please connect your sensor.
Solution 1: Plug in one sensor
Plugging in a single Rift sensor will prevent this error from being displayed. You can plug the sensor into a USB 2.0 port, and you do not need to run Oculus Setup if you do this (even if prompted). It's also recommended to cover up the sensor to avoid having it affect tracking unexpectedly.
Solution 2: Use controller to close window
You can use a regular controller, such as an XBOX controller, to press the OK button. With the controller plugged in, a crosshair will appear in front of your gaze. Simply look at the OK button and press the primary button (A, a trigger, etc). This must, unfortunately, be done each time the Oculus platform is started.
You can also use an Oculus Touch to close the window. However, you'll need to aim the controller at the OK button, which is difficult without the controller being tracked by sensors as the rotation is likely to be quite off.