A change in the order of amino acids can completely change the entire structure of the protein.
The shape of the protein is the result of how amino acids react to each other and the environment. Amino acids are all very similar in structure, but each has an individual R group that has its own properties, such as polar, non polar, able to form disulphide bonds, etc. that dictates how it will interact (ex. non polar AAs will be on the inside of the protein while polar will be on the outside, two AAs that like to form disulphide bonds will gravitate toward each other).
If one of these amino acids is misplaced, the protein is likely to be shaped a completely different way which often hinders its functionality.