Magento2 – Adding a Custom Customer Attribute

You have to keep below code inside your UpgradeData.php file,

<?php
/**
 * Copyright © 2015 Clounce. All rights reserved.
 * See LICENSE.txt for license details.
 */

namespace {Vendor}\{Module}\Setup;

use Magento\Framework\Setup\UpgradeDataInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;
use Magento\Eav\Setup\EavSetup;
use Magento\Eav\Setup\EavSetupFactory;
use Magento\Customer\Model\Customer;

class UpgradeData implements UpgradeDataInterface
{
    public function __construct(EavSetupFactory $eavSetupFactory)
    {
        $this->eavSetupFactory = $eavSetupFactory;
    }

    /**
     * Installs data for a module
     */
    public function upgrade(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
    {
        $eavSetup = $this->eavSetupFactory->create(['setup' => $setup]);

        $attributes = [
            'test'=> [
                "type"     => "varchar",
                "label"    => "Your custom label",
                "input"    => "text",                
                'sort_order' => 100,
                "required" => false,
                'position' => 100,
                'system' => false,                
                //'validate_rules' => 'a:2:{s:15:"max_text_length";i:255;s:15:"min_text_length";i:1;}',
            ],
        ];

        foreach ($attributes as $code => $options) {
            $eavSetup->addAttribute(
                Customer::ENTITY,
                $code,
                $options
            );
        }

        $this->installCustomerForms($eavSetup);
    }

    /**
     * Add customer attributes to customer forms
     */
    public function installCustomerForms(EavSetup $eavSetup)
    {
        $customer = (int)$eavSetup->getEntityTypeId(\Magento\Customer\Model\Customer::ENTITY);
        /**
         * @var ModuleDataSetupInterface $setup
         */
        $setup = $eavSetup->getSetup();

        $attributeIds = [];
        $select = $setup->getConnection()->select()->from(
            ['ea' => $setup->getTable('eav_attribute')],
            ['entity_type_id', 'attribute_code', 'attribute_id']
        )->where(
            'ea.entity_type_id IN(?)',
            [$customer]
        );
        foreach ($eavSetup->getSetup()->getConnection()->fetchAll($select) as $row) {
            $attributeIds[$row['entity_type_id']][$row['attribute_code']] = $row['attribute_id'];
        }

        $data = [];

        $attributes = [
            'test'=> [
                "type"     => "varchar",
                "label"    => "Your custom label",
                "input"    => "text",                
                'sort_order' => 100,
                "required" => false,
                'position' => 100,
                'system' => false,                
                //'validate_rules' => 'a:2:{s:15:"max_text_length";i:255;s:15:"min_text_length";i:1;}',
            ],
        ];

        foreach ($attributes as $attributeCode => $attribute) {
            $attributeId = $attributeIds[$customer][$attributeCode];
            $attribute['system'] = isset($attribute['system']) ? $attribute['system'] : true;
            $attribute['visible'] = isset($attribute['visible']) ? $attribute['visible'] : true;
            if ($attribute['system'] != true || $attribute['visible'] != false) {
                $usedInForms = ['customer_account_create','adminhtml_customer','checkout_register','customer_account_edit','adminhtml_checkout'];

                foreach ($usedInForms as $formCode) {
                    $data[] = ['form_code' => $formCode, 'attribute_id' => $attributeId];
                }
            }
        }

        if ($data) {
            $setup->getConnection()->insertOnDuplicate($setup->getTable('customer_form_attribute'), $data);
        }
    }
}

Remove var folder from root and run command,

php bin/magento setup:upgrade

and

php bin/magento indexer:reindex

Check again. Its working.

Try that code to save custom attribute of customer

$customerRepository = $this->objectManager->get('Magento\Customer\Api\CustomerRepositoryInterface');

$customer = $customerRepository->getById('customer_id');

$customer->setCustomAttribute("is_checked",'0');

$customerRepository->save($customer);