Error T300 means the water leak sensor was not installed. The following steps can help you check the issue.
Check to ensure the water leak sensor is installed.
Check wiring connections to sensors from the board.
Check resistance to the sensor to determine whether the sensor or board should be replaced (See ohm reading chart for correct readings).
Zwell is dedicated to getting homes closer to net zero through the installation of new energy-efficient hybrid water heaters. We are not currently equipped to do service or repair visits but we want to be as helpful as possible. Feel free to reach out to us here or for more information, you can contact the Rheem Hybrid Technical Services Department at 800-995-0982.