getting error: Cannot read property state of undefined

There are a few things that are technically wrong in terms of React code implementation.

Firstly, With ES6 style of writing a class, any function that needs to access the Class properties need to be explicitly binded. In your case you need to bind the handleSubmit function using arrow function of or binding in constructor.

See this answer for more details: Why and when do we need to bind functions and eventHandlers in React?

Secondly: You have your async request set up in the componentWillMount function and in the success response of it, you are setting state. However using setState in componentWillMount is triggered after the component is rendered so you still need to have an undefined check. You should instead make use of componentDidMount lifecycle function for async requests.

Check this answer on whether to have AJAX request in componentDidMount or componentWillMount

Third: setState is asynchronous and hence logging the state values after the setState function won’t result in the correct output being displayed. Use the setState callback instead.

See these answers for more details:

calling setState doesn’t mutate state immediately

When to use React setState callback

Code:

export default class Update extends Component {
    constructor(props) {
        super(props);

        this.state = {
          updateClothingItem: {}
        };
    }

    componentDidMount() {
        fetchClothingItem(this.props.match.params.postId)
        .then(data => {
        this.setState(state => {
          state.updateClothingItem = data;
          return state;
        });
        console.log("data", data);

        //HERE IT IS RETURNING EXPECTED DATA       

        console.log("this.state.updateClothingItem",this.state.updateClothingItem)    
          }) // this statement will not show you correct result since setState is async 
        .catch(err => {
            console.error("err", err);
        });
    }

    handleSubmit = (data) =>  { .    // binding using arrow function here

        console.log("this.state.updateClothingItem", this.state.updateClothingItem);
            updateClothingItem(this.state.updateClothingItem.id, data); this.props.router.push("/update");
    }
    render() {
        return (
        <div>
        <FormUpdate
          //onSubmit={this.handleSubmit.bind(this)}
          id={this.state.updateClothingItem.id}
          name={this.state.updateClothingItem.name}
          sleeveLength={this.state.updateClothingItem.sleeveLength}
          fabricWeight={this.state.updateClothingItem.fabricWeight}
          mood={this.state.updateClothingItem.body}
          color={this.state.updateClothingItem.color}
        />
        <button
          type="submit"
          onClick={this.handleSubmit}
          className="addItemButton"
        >
        Button
        </button>
      </div>
    );
  }
}