DIAGNOSIS That's the printer's generic 'something is wrong' alert. It's usually a paper jam, but it can also mean a carriage error or an ink cartridge issue.
FIX First, power the printer completely off and unplug it for 30 seconds. Open the front cover and the paper tray. Visually and physically check for any tiny scraps of paper along the entire paper path, especially in the rear feed area. Gently slide the print head carriage from side to side to make sure it's not stuck. Plug it back in and power it on.
IF THAT DIDN'T WORK The error could be a failing ink cartridge or one that's not seated properly. Remove and firmly reseat all the cartridges. If the red light persists after that, try replacing the black cartridge first, as a depleted or faulty one can trigger this.
HEADS UP When checking for jams, never force anything. If you see a piece of paper stuck, pull it slowly in the direction the paper normally travels to avoid tearing it and leaving a fragment behind.