Hello Andrzej,
Thanks for your question. While creating child theme there may be something error. So please create a child theme with proper codes and standards. And if you create a function on the child theme, please give the new name of function. The function you are creating above is already created on the parent theme, so this error is occur. Please have a look here for more details about child theme.
If the problem is not solved, please let us know.
Thanks