Can I use non static variable in static method in java?

You seemed to be confused w.r.t instance variables and local variables.

You can always declare a "local variable" inside a static method. main() for example is a static function and we always declare variables inside it.

So your creation of a variable "in" of type Scanner inside input() function is perfectly fine.

However, you "cannot" access instance variables and instance methods from static methods.

This post on stack overflow gives a full and complete answer: Can non-static methods modify static variables

As far as your code is concerned, there's a minor error in the code. The function call to read an integer is "nextInt" and not "Nextint". Java generally uses camel-case to define all its methods. So be careful with the method usage.

The modified code should be this:

class Operator
{
    static int a;
    public static void input() { 
        Scanner in=new Scanner(System.in); 
        System.out.println("Enter the number:"); 
        a=in.nextInt(); //this is nextInt and NOT Nextint
    }

    public static void output() { 
        System.out.println("Number is:" + a); 
    }

    public static void main (String[] args) throws java.lang.Exception
    {
        // your code goes here
        input();
        output();

    }
}

short answer - NO reason is simple too, that is - it will violates the definition of static i.e. accessible in other class without creating a object(also called instance) of the class.

But, what if we try to do static variable in a non-static method ? In that case, YES we can do that. Because we have to create a instance (object) of the class to use that method. So, that doesn't violates the definition.