• your first line (inside the function) clearly isn't doing what you think it does (btw, bin_a has never been defined before so... => !??)
• and reverse_inplace! isn't a function of the ruby String class, afaik (careful about the language version too: on CW, you're limited to ruby 2.5.0)

edit: btw, you're not supposed to have an argument to that function (you need to learn a bit more about class/OOP design and how to declare methods in such a kind of situation)

You can read about code formatting on CodeWars wiki.

You might want to try easier kata first, then. Try going through some more whites, 8 kyu and 7 kyu, and when you feel comfortable with these, go to more difficult ones (easy yellows), and so on.