[![enter image description here](https://i.stack.imgur.com/5IEFW.jpg)](https://i.stack.imgur.com/5IEFW.jpg)

I have changed the derailleur on my daughters back, bought like for like.

Now fitted.

When I try to pedal the derailleur, it flicks backwards and the bike won’t pedal forward, it remains stuck. 

Any advice would be greatly appreciated.