A woman approached in an alleyway in Royston on Saturday was asked to hand over her phone and money.
Officers are appealing for anyone with information about the attempted robbery to get in contact.
The incident happened between 4.15pm and 4.45pm on Saturday, in the alleyway that connects Houseman Avenue to a field.
The victim was approached by a man who was sitting on a black sprayed BMX bike.
The offender asked the woman to hand over her phone and money but she refused, after which he rode off in the field in the direction of Tesco.
He is described as white, aged around 13 to 15, of slim build, around 5ft 7in tall and had a London accent.
He was wearing dark jogging bottoms, a dark hoodie with the hood up and a snood with a skeleton jaw on it.
Anyone with information can contact Det Con Simon Goodship by emailing simon.goodship@herts.pnn.police.uk.
You can also provide information online at www.herts.police.uk/Report, or call the non-emergency number 101, quoting 41/51645/19.
Alternatively, contact Crimestoppers anonymously on 0800 555 111 or via their untraceable online form at www.crimestoppers-uk.org.
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