Isn't it possible to remove an accidentally submitted solution?

I bet you don't like Python.

Usually, when a test fails, the log tells you "why" it failed (it is descriptive enough, usually meaning in general that for certain inputs a given result is expected).

You must understand the requirements.
If you understand the requirements, then you can add your own tests (it is highly encouraged to do so).
If you cannot add more tests, then probably you didn't understand the requirements at all.

If you fully understand the requirements, then you will be able to write a function that passes all tests (as long as these are correctly written).