Trying to tighten the chain tensioner on a razor scooter

That is a complicated system. Just take it into a local cycle shop.

https://razor.com/products/electric-scooters/ecosmart-metro-electric-scooter/#

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Check to see if the chain is moving when you apply throttle. If the chain is not moving but you can hear the motor working then it is likely the chain has come off the motor sprocket. If this is the case then follow the steps in the user manual to refit the chain and adjust the tension.

Theres an 8mm nut holding it onto the pin that goes thru the frame.
Take that nut off and remove the tensioner. Put some grease on the new pin where the tensioner goes. Put the end of the new spring into the hole in the frame next to the pin hole. Hold the tensioner in place and put the pin thru and the nut back on. You should be able to get the chain back on the sprocket without loosening the axle nuts. Hope this helps.

These are the instructions for adjusting the chain. They are taken directly from the manual available at http://ep.yimg.com/ty/cdn/razorama/e100e200e300manual.pdf

To adjust the chain:
1. Loosen the axle
2. Loosen the brake anchor (this is attached with the small bolt in the long slot on the left side of the rear fork)
3. Tighten the tension adjusters on the axle 1/8 to 1/4 turn to tension the chain. Tighten both adjusters the exact same amount to maintain the wheel alignment.
Note: this system of adjusters is common to motorcycles and BMX
bicycles so I your are not familiar with it, if you know anyone who is,
they would not have a problem adjusting it for you.
Chain tension: The chain should be "just not-quite snug" in other
words, not taut or sloppy loose. BE CAREFUL NOT TO ADJUST TOO TIGHT. The tensioning system can easily impart too much tension and snap the chain or bend the motor output shaft. Turn the adjusters 1/8 to 1/4 turn at a time and recheck the tension each time.
4. Once the tension is good, tighten the axle and brake anchor bolt. Test run the scooter. Readjust as needed.

tensioner should fit on axle or behind the axle on the edge of thew fork where hardware should be mounted to adjust tensioner

On the Razor E200 check the chain is fully tightened & lubricated & you will be amazed what incorrect tyre pressures will do (36psi) to the power of the scooter.

First your going to remove the deck.
Take off the chain tensioner
Next remove the motor and put the chain around the gear
Put the motor back on the scooter with the chain on
Then take the rear wheel off and put the chain into the gear.
Put the rear wheel back on along with the chain tensioner and deck
Your done!

Note: the reason why I say you have the remove the motor and wheel is the get the chain on. So you may only have to partial remove the motor or tires to get the chain on.

If the chain is too long than shorten it by using a chain breaker. Or you don't have a chain breaker take the scooter to a bike or more preferable a electric scooter shop and have them do it. The problem could also be your chain tensioner check to see if it's tightening the chain.