What does the `ValueError: if using all scalar values you must pass an index` error in Pandas indicate?

This error occurs when you try to use `loc` or `iloc` to select a subset of a DataFrame when you're providing a scalar value (like a single number or string) instead of a list, array, or a boolean array. Pandas expects an index or position to select rows or columns, and when you only have a single value, it doesn't know which row or column you're referring to.