I have some electric scooter problems... Please I can use all the help I can get.?

When I keep In the charger It will turn on but cant run whike I'm on it. If I lift up the back wheel it will run for about 15-20 seconds while it goes from full to 1/2 way to 1/3 and then empty. Can anyone tell me what might be wrong?

Sounds like the battery is worn out and can't be recharged any more. You should replace the battery...