Hello @Pierre_BEJIAN
The problem may be that your notebook is corrupted, it the implementation is correct,
You can save your current notebook and then reset your workspace. After that, you can copy your code to a fresh new notebook and try resubmitting.
I hope this link can help you to carry out the reset process.
If it is the colab notebook there may be an error in your implementation. Can you send me a private message with your notebook. I can see if I can help out.
Thanks.