You need to use javascript to achieve this

1. override the WooCommerce product template to add a custom buy now button
2. add click event on that buy now button and check if any variation option is selected or not.
3. IF not give alert to the user
4. If yes send an ajax call to add that product to the cart
5. on success redirect the user to the checkout page

let's assume you give the button id as wc_buynow

$(document).on('click','#wc_buynow',function(){
    let is_valid = 1;
    $('select.wc-default-select').each(function(i){
        if($(this).val() == ''){
            let id = $(this).attr('id');
            let attr_name = $("label[for='"+id+"']").text();
            alert(attr_name+" is required!");
            is_valid = 0;
        }
    });

    if(is_valid){
      // send ajax call to add product in cart and on success redirect to checkout
    }

    
})