Type 'void' is not assignable to type '((event: MouseEvent<HTMLInputElement>) => void) | undefined'

In your code this.fetchData("dfd") you are calling the function. The function returns void. void is not assingable to onClick which expects a function.

Fix

Create a new function that calls fetchData e.g. onClick={() => this.fetchData("dfd")}.

More

This is a very common error prevented by TypeScript

You could also do something like

fetchData = (val: string) => (event: any) => {
  alert(val);
};

Alternatively, you can set a type for your event, such as React.MouseEvent. You can read more about it here.

With Functional Components, we use React.MouseEvent and it clears things up...

const clickHandler = () => {
  return (event: React.MouseEvent) => {
    ...do stuff...
    event.preventDefault();
  }
}