Most often this might be caused by loose or incorrect connections, or the product is designed for a different application. First check the installation manual to be sure all connections are correct, and that the device is designed to supply the voltage for your application. If all connections are correct, check the power supplies. Are the batteries charged sufficiently for the load?
If it is a solar charge controller, are the solar modules connected correctly and are they receiving good sunlight to operate?
Always best to review each component manual to ensure a safe and successful installation. The most common problem we see is customers bought a solar controller designed for 12VDC battery, but they have the batteries wired for 24VDC or vice versa.
The second most common problem is that the batteries are low voltage, and the controller shuts off the load output (Low Voltage Detect Function). The controllers will turn off the load at around 11V (22V for 24V battery systems).