Javascript modules import fails

One of the great things about modules is that top level declarations, etc., in them don't create globals. One of the bad things about onxyz-attribute-style event handlers is that the functions you call with them have to be globals. Your sayHi isn't a global, so onload="sayHi('Manish')" fails because it doesn't have access to sayHi.

Which is a good thing.

Instead, just call the function from main.js:

import { sayHi } from './say.js';
sayHi('MK');
sayHi('Manish');

Because module scripts are automatically deferred until the end of HTML processing, you know that won't happen until all the HTML is loaded. This is covered by a great graphic in this section of the spec, duplicated here:

If you want to wait longer, until the load event (which doesn't fire until all images and such are loaded), use a modern event handler to do that:

import { sayHi } from './say.js';
sayHi('MK');
window.addEventListener("load", () => {
    sayHi('Manish');
});

If you need information from the element you hooked the event on, use a traditional function and access the element as this, or accept the event parameter and use event.currentTarget (you can also use event.target for the element that the event targets, which could be within the element you hooked the event on). So for instance, suppose you have:

<button type="button" data-name="Manish" id="btn-say-hi">

you could have:

import { sayHi } from './say.js';
document.getElementById("btn-say-hi").addEventListener("click", function() {
    sayHi(this.getAttribute("data-name"));
});

Also note that as Vikas Saini pointed out your say.js is using a string literal instead of a template literal (and although he/she didn't mention it, also has the wrong syntax for a substitution), so you'll actually see the text Hello { $user } instead of seeing Hello MK or Hello Manish. Either use a template literal with the correct form of substitution (${user}, not { $user }):

export function sayHi(user) {
    alert(`Hello, ${user}`);
}

or simple string concatenation:

export function sayHi(user) {
    alert("Hello, " + user);
}