Please check all your cable connections, specially that of the sensor.

Your darkroom light should be switched off by the Heiland console (socket safelight) during the measurement. Because in continuous mode there is a dynamic measurement of the light and the gradation, you should measure by the working aperture.

If the error is "Check contrast" the measured contrast is out of the print range of the selected paper. If you proceed, you will loose details in highlights and the result will be a grey print, because the paper grade could not be fitted to your negative.

Prevent measurement on very small spots like lamps or so.

Best regards,