Ninebot Max Error Code 19
"Battery Voltage detection is abnormal, Please check the Battery, Control Board and related connections"
This error is thrown when the battery believes it's under the correct voltage.
The battery likely needs to be charged, plug in the charge and see if it stays red or oscillates between green to red or stays solid green.
If it is solid red, nothing more is needed, let it charge.
If it is flashing between green and red, leave it for an hour or so and it should begin normally charging a short while later.
If it stays solid green, your battery may need to be replaced.