The red light means your Flowie does not have a good signal, which your sensor will need in order to report its data.
To improve the signal, try adjusting the antenna and wait a couple of minutes while it tries to connect from the new position. Repeat as necessary. For a new Flowie installation, it may take a few minutes before the light turns green.
If you are in an area with low or intermittent cellular reception, you will need an extension antenna to improve reception in your area.