Set max or min cart weight - https://www.damiencarbery.com/2018/06/set-max-or-min-cart-weight/

limit-cart-weight-increments.php

<?php
/*
Plugin Name: Limit Cart Weight to increments
Plugin URI: https://www.damiencarbery.com/
Description: Prevent checkout unless cart is above a minimum weight and increments of a weight.
Author: Damien Carbery
Author URI: https://www.damiencarbery.com
Version: 0.1
*/


add_action( 'woocommerce_check_cart_items', 'dcwd_cart_weight_min_and_increments' );
function dcwd_cart_weight_min_and_increments() {
	// Set the minimum cart weight.
	$minimum_weight = 6;
	$increments = 6;

	// Get the cart's total weight
	$cart_contents_weight = WC()->cart->get_cart_contents_weight();

	// Compare values and add an error if less than the minimum weight
	if ( ( $cart_contents_weight % $increments != 0 ) || ( $cart_contents_weight / $minimum_weight < 0 ) ) {
		// Display our error message
		wc_add_notice( sprintf('<strong>Your order must be at least %s%s before checking out.</strong><br />Current cart weight: %s%s',
			$minimum_weight, get_option( 'woocommerce_weight_unit' ),
			$cart_contents_weight, get_option( 'woocommerce_weight_unit' ) ),
			'error'	);
	}
}

limit-lower-cart-weight.php

<?php
/*
Plugin Name: Limit Cart Weight
Plugin URI: https://www.damiencarbery.com/
Description: Prevent checkout unless cart is above a certain weight.
Author: Damien Carbery
Author URI: https://www.damiencarbery.com
Version: 0.1
*/

// Based on: https://www.cloudways.com/blog/set-purchase-limits-on-woocommerce/
add_action( 'woocommerce_check_cart_items', 'dcwd_set_minimum_cart_weight' );
function dcwd_set_minimum_cart_weight() {
	// Set the minimum cart weight.
	$minimum_weight = 5;
	// Get the cart's total weight
	$cart_contents_weight = WC()->cart->get_cart_contents_weight();
	// Compare values and add an error if less than the minimum weight
	if( $cart_contents_weight < $minimum_weight  ) {
		// Display our error message
		wc_add_notice( sprintf('<strong>Your order must be at least %s%s before checking out.</strong><br />Current cart weight: %s%s',
			$minimum_weight, get_option( 'woocommerce_weight_unit' ),
			$cart_contents_weight, get_option( 'woocommerce_weight_unit' ) ),
			'error'	);
	}
}

limit-upper-cart-weight.php

<?php
/*
Plugin Name: Limit Cart Weight
Plugin URI: https://www.damiencarbery.com/
Description: Prevent checkout unless cart is above a certain weight.
Author: Damien Carbery
Author URI: https://www.damiencarbery.com
Version: 0.1
*/

// Based on: https://www.cloudways.com/blog/set-purchase-limits-on-woocommerce/
add_action( 'woocommerce_check_cart_items', 'dcwd_set_maximum_cart_weight' );
function dcwd_set_maximum_cart_weight() {
	// Set the maximum cart weight.
	$maximum_weight = 5;
	// Get the cart's total weight
	$cart_contents_weight = WC()->cart->get_cart_contents_weight();
	// Compare values and add an error if less than the maximum weight
	if( $cart_contents_weight > $maximum_weight  ) {
		// Display our error message
		wc_add_notice( sprintf('<strong>Your order must be less than %s%s before checking out.</strong><br />Current cart weight: %s%s',
			$maximum_weight, get_option( 'woocommerce_weight_unit' ),
			$cart_contents_weight, get_option( 'woocommerce_weight_unit' ) ),
			'error'	);
	}
}