50cc stop working after three miles, brand new, delivered today

Flat battery. Unless the battery is fully 12.2 - 12.6 volts, it won't start with the electric start. If the voltage goes below 11 volts, there is not enough power for fuel injection and ignition assuming your mystery scooter has fuel injection. Connect it to a battery charger for 8 hours.

Engines only need three things to run.

Fuel in the correct amounts
A Spark at the right time
Compression

Check the compression. Make sure the fuel is reaching the spark plug.

the sym scooter is shown as a oil in the sump requirement with a dipstick to check oil level
that will mean that only straight 91 octane fuel is put in the tank
go google and type in--sym 50cc motor scooter user manual -- and there is a page with a pdf version download of all the required knowledge for the scooter
the site is --www.49 ccscoot.com

CHECK VOLTAGE WITH VOLT METER AND GAS/OIL MIX COULD BE RUNNING ROUGH

You should put in approx 900 ml of 5W30 or 10W30 engine oil. Make sure the oil level on the dipstick doesn't get onto the plastic. As long as it's on the metal, it's good.

Keep changing parts until you find the solution! No, just kidding. Many will do this if they don't understand how an engine works. It sounds like it may be a hung choke. Engines need extra fuel when cold, and die when warm if they are still getting that extra fuel. Also, low oil level will give the same indication. There are many variables.

you need to have some fuel in the system over night as when it is shipped it is completly dry when this is done it should start with out any problems if it still has the same problem then you will need to charge the battrie

the recommended mix is 40:1. So for 5 liters of fuel you're looking at 4.2 ounces of oil.

Check your jets in your carbs and also could be bad fuel. Poor some sea foam in it and some new fuel and check to see if the inline filter is plugged and check your air filter as well.