Comments - Field 'xxxx' doesn't have a default value

fax, company, and org_sponsor are also giving the error.

MyISAM storage

-- phpMyAdmin SQL Dump
-- version 4.6.6deb4+deb9u1
-- https://www.phpmyadmin.net/
--
-- Host: localhost
-- Generation Time: May 10, 2021 at 09:37 AM
-- Server version: 10.3.27-MariaDB-0+deb10u1-log
-- PHP Version: 7.3.27-1~deb10u1

SET SQL_MODE = "NO_AUTO_VALUE_ON_ZERO";
SET time_zone = "+00:00";


/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;
/*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;
/*!40101 SET NAMES utf8mb4 */;

--
-- Database: `opencart`
--

-- --------------------------------------------------------

--
-- Table structure for table `customer`
--

CREATE TABLE `customer` (
  `customer_id` int(11) NOT NULL,
  `customer_group_id` int(11) NOT NULL,
  `store_id` int(11) NOT NULL DEFAULT 0,
  `firstname` varchar(64) NOT NULL,
  `lastname` varchar(64) NOT NULL,
  `email` varchar(96) NOT NULL,
  `telephone` varchar(32) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
  `fax` varchar(32) NOT NULL,
  `password` varchar(40) NOT NULL,
  `salt` varchar(9) NOT NULL,
  `cart` text DEFAULT NULL,
  `wishlist` text DEFAULT NULL,
  `newsletter` tinyint(1) NOT NULL DEFAULT 0,
  `address_id` int(11) NOT NULL DEFAULT 0,
  `custom_field` text NOT NULL,
  `ip` varchar(40) NOT NULL,
  `status` tinyint(1) NOT NULL,
  `approved` tinyint(1) NOT NULL,
  `safe` tinyint(1) NOT NULL,
  `token` varchar(255) NOT NULL,
  `date_added` timestamp NOT NULL DEFAULT current_timestamp(),
  `wmmw_id` varchar(64) NOT NULL,
  `photo` varchar(50) NOT NULL,
  `voice` varchar(20) NOT NULL,
  `voice2` varchar(20) NOT NULL,
  `manager` int(2) NOT NULL,
  `rank` varchar(20) NOT NULL,
  `rank_icon` varchar(20) NOT NULL,
  `pagetext_en` varchar(300) NOT NULL,
  `pagetext_fr` varchar(300) NOT NULL,
  `pp_email` varchar(96) NOT NULL,
  `validated` varchar(12) NOT NULL,
  `visitor` int(2) NOT NULL,
  `candidate` int(2) NOT NULL,
  `reseller` varchar(10) NOT NULL DEFAULT 'No',
  `coop_page` int(2) NOT NULL,
  `wmmw_domain` varchar(128) DEFAULT NULL,
  `volunteer` int(2) NOT NULL DEFAULT 0,
  `level` varchar(30) DEFAULT NULL,
  `authorized` varchar(20) NOT NULL,
  `target` decimal(12,2) NOT NULL,
  `members` int(12) NOT NULL,
  `charter` varchar(50) NOT NULL,
  `raffle` varchar(25) NOT NULL,
  `raffle2` varchar(25) NOT NULL,
  `raffle3` varchar(25) NOT NULL,
  `sponsor_id` varchar(16) NOT NULL,
  `coordinator_level` varchar(25) NOT NULL,
  `beta` varchar(20) DEFAULT NULL,
  `org_sponsor` varchar(16) NOT NULL,
  `coordinator` varchar(12) NOT NULL DEFAULT 'No',
  `angel` varchar(12) NOT NULL,
  `apprentice` varchar(12) NOT NULL,
  `angel_id` varchar(40) NOT NULL,
  `comptroller` varchar(12) NOT NULL,
  `deposit` varchar(20) NOT NULL,
  `gencomp` varchar(12) NOT NULL,
  `comptroller_id` varchar(20) NOT NULL,
  `gencomp_id` varchar(20) NOT NULL,
  `gencomp_id2` varchar(20) NOT NULL,
  `coordinator_id` int(4) NOT NULL DEFAULT 0,
  `p2` varchar(30) NOT NULL,
  `timezone` varchar(30) NOT NULL,
  `raffle4` varchar(12) NOT NULL,
  `raffle5` varchar(12) NOT NULL,
  `primary_raffle` varchar(12) NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

--
-- Indexes for dumped tables
--

--
-- Indexes for table `customer`
--
ALTER TABLE `customer`
  ADD PRIMARY KEY (`customer_id`);

--
-- AUTO_INCREMENT for dumped tables
--

--
-- AUTO_INCREMENT for table `customer`
--
ALTER TABLE `customer`
  MODIFY `customer_id` int(11) NOT NULL AUTO_INCREMENT;
/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */;
/*!40101 SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS */;
/*!40101 SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION */;

Thanks Ian... But how can I specify a default value for a field that is primary and auto-increment, such as the customer_id field?

Primary key auto-increments can be made null:

ALTER TABLE customer MODIFY customer_id INT(11) NULL AUTO_INCREMENT;

but not given a DEFAULT value, as the AUTO_INCREMENT takes care of that:

ALTER TABLE customer MODIFY customer_id INT(11) NOT NULL DEFAULT '0' AUTO_INCREMENT;
ERROR 1067 (42000): Invalid default value for 'customer_id'

You shouldn't get the "Field 'xxxx' doesn't have a default value" error on an auto-increment, so if you are, it would help to find the queries resulting in the problem as well as the table state at the time of these errors.