Usually "ERR" on the monitor means a movement was detected while measuring blood pressure.
We always recommend placing the cuff 1" above the elbow, with the air hose pointing down and aligned towards the forearm. Also, it's recommended to sit comfortably for 5 minutes with both feet on the floor, and the hands resting on a flat surface (table works great!)
If your monitor is manufactured by A&D, and you got the "ERR" message on it, please measure your blood pressure one more time, making sure you sit still and don't move or talk while measuring your blood pressure, considering the above recommendations. Let us know if that works!