python least_square `bounds` must contain 2 elements.

The error message "bounds must contain 2 elements" usually occurs when you are using the `least_squares` function from the `scipy.optimize` module in Python and you have not specified the bounds correctly.

The `bounds` parameter is used to specify the minimum and maximum values for each variable in the optimization problem. It should be a sequence of length 2, where the first element is the lower bound and the second element is the upper bound.

To fix the error, make sure that you have provided the correct number of elements for the `bounds` parameter. For example, if you have two variables in your optimization problem, you should provide a list of two tuples, each containing the lower and upper bounds for that variable:

bounds = [(0, 1), (0, 2)]

This sets the lower bound of the first variable to 0 and the upper bound to 1, and sets the lower bound of the second variable to 0 and the upper bound to 2.