DIAGNOSIS Error 13 means "Main Brush stuck." It's not necessarily that your brush is tangled, but that the robot's motor is detecting too much resistance when it tries to spin.
FIX 1) Flip the robot over. 2) Remove the main brush cover by pressing the two tabs. 3) Take out the main brush itself and clear any hair, string, or debris wrapped around the ends of the brush or inside the bearing housing. 4) Also, check the side brush—if it's mangled or caught on something, it can cause enough drag to trigger the error. Clear that too. 5) Reinstall everything, making sure the brush spins freely by hand before you click the cover back on.
IF THAT DIDN'T WORK The issue might be the brush cover not being fully snapped into place, which misaligns the brush. Remove and firmly reseat it, listening for a solid click on both sides. A worn-out main brush with bent fins can also cause constant resistance and needs replacing.
HEADS UP Run your fingers along the entire brush chamber with the brush removed. Sometimes a staple or small rock gets lodged in there, causing friction you can't see. That'll give you the error even with a brand new brush.