Diagnosis
Solid red means a system halt, usually triggered by a failed initial self-check or a pressure sensor fault. It's the machine's way of saying something is too wrong to proceed.
How to Fix It
First, power cycle it. Unplug the unit from the wall outlet for a full minute. While it's off, also turn the main water supply valve off and back on. Plug it in and listen. You should hear the pump kick on and the system go through a startup flush. The light should cycle to blue.
If That Didn't Work
Check the water supply itself. If your main house pressure is too low (below 40 PSI), the unit won't start. Also, make sure the pre-filter (the first stage) isn't completely clogged, as that can starve the system.
Heads Up
If it goes red again immediately after this, there's a good chance one of the internal solenoid valves is stuck or the pump has failed. That's a call-to-support situation.