How can I prevent subscriber role from bidding?

To accomplish that you can use code snippet below:

// snippet that prevents bidding based on user role

add_filter( 'auctions_for_woocommerce_before_place_bid_filter' , 'afw_prevent_bid_by_user_role' );

function afw_prevent_bid_by_user_role() {

    $current_user = wp_get_current_user();

    if ( in_array ( 'subscriber', $current_user->roles ) ){
        wc_add_notice( esc_html__('Your user role cannot bid.', 'auctions-for-woocommerce'), 'error' );
        return FALSE;
        
    } else {

        // user can place a bid
        return TRUE;
    }
}