If you see a message like this, your OT-2 is having trouble communicating with the pipette that you just attached.
First, try unplugging the white connector tab from the pipette and re-inserting it. Ensure that the connector is plugged in securely. Then, click Have robot check connection again.
If the pipette still cannot be detected, there may be a problem with the pipette, or with one of the OT-2's cables. To narrow things down:
See if the pipette can be detected in the other mount, leaving the original mount empty.
See if a different pipette can be detected in the same mount.
And contact Opentrons Support for assistance.