Assuming Mr. 66 is psychic and you truly have single speed bike, ie. one with a coaster brake, then I'm with the others in suspecting a slack chain.
This is easily fixed, by loosening the rear wheel and sliding it back.
Just be aware of 2 concerns as you do this.
1- you must hold the wheel centered between the chainstays (front of wheel, behind pedals)
2- DO NOT pull out all the slack. The chain needs some slack to work right. Snug the wheel and push the lower loop up and down in the middle. It needs about 1/4" or so. Then check it's centered & tighten the wheel nuts completely.
You can also search tutorials for installing a coaster brake wheel.
Last edited by FBinNY; 04-28-24 at 01:48 PM.