Insert HTML Page in WordPress Page

You can’t because it’s a non-secure page http but you’re loading into an https page. It’s called Mixed Content.

Adjust your protocol to // and hope it renders over https.

<iframe src="https://creditsmart.in/wp-content/themes/voice/emicalc.html" name="frame1" scrolling="auto" frameborder="no" align="center" height = "300px" width = "100%">
</iframe>

( iframe code modified from @Ittikorn’s answer )

If the file is in your same theme directory it’s possible to create a shortcode:

[emicalc]

Then render the local file:

function emicalc__shortcode( $atts ) {

    $content = file_get_contents( get_stylesheet_directory() . '/emicalc.html');
    return $content;
}

add_shortcode( 'emicalc', 'emicalc__shortcode' );

If it’s remote you might be able to use wp_remote_get() instead of file_get_contents() but it requires you pull the contents using the server instead of allowing the client to do it.

There is no other trick / https can only load https / http can always load https.