Error in creating custom customer attribute in magento 2

You could not create a customer attribute using magento schema files both InstallSchema and UpgradeSchema.Need to use data files InstallData and UpgradeData.

Because of in order create an attribute you should inject Magento\Customer\Setup\CustomerSetupFactory on __construct function of your class and Magento\Customer\Setup\CustomerSetup main class of this factory class.

But this class Magento\Customer\Setup\CustomerSetup class injects at Magento\Framework\Setup\ModuleDataSetupInterface on it's __construct function.

And as you use Magento\Framework\Setup\InstallSchemaInterface and Magento\Framework\Setup\SchemaSetupInterface.

So it cannot be possible to get interface Magento\Framework\Setup\ModuleDataSetupInterface at Magento\Customer\Setup\CustomerSetup.

And you can only use Magento\Framework\Setup\ModuleDataSetupInterface on data installdata and upgradedata file mean data installer file.

Solution is that you should use UpgradeData.php instead of UpgradeSchema.php

Installer should be

<?php

namespace  Namespace\Modulename\Setup;

use Magento\Customer\Model\Customer;
use Magento\Customer\Setup\CustomerSetup;
/* irrelevant */
#use Magento\Framework\Setup\UpgradeSchemaInterface;
use Magento\Framework\Setup\InstallSchemaInterface;
use Magento\Framework\Setup\ModuleContextInterface;
/* irrelevant */
#use Magento\Framework\Setup\SchemaSetupInterface;
/* add this */
use Magento\Framework\Setup\UpgradeDataInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;

class UpgradeData implements  UpgradeDataInterface
{
     private $customerSetupFactory;

    public function __construct(\Magento\Customer\Setup\CustomerSetupFactory $customerSetupFactory)
    {
        $this->customerSetupFactory = $customerSetupFactory;
    }
    public function upgrade(ModuleDataSetupInterface $setup,
                            ModuleContextInterface $context){
        $setup->startSetup();

            if (version_compare($context->getVersion(), '0.0.4') < 0) {
            /** @var CustomerSetup $customerSetup */
            $customerSetup = $this->customerSetupFactory->create(['setup' => $setup]);
            $customerSetup->addAttribute(
                Customer::ENTITY,
                'example1',
                [
                    'label' => 'Example Attribute',
                    'type' => 'int',
                    'default' => '0',
                    'visible' => true,
                    'required' => 0,
                    'system' => false, // <-- important, otherwise values aren't saved.
                                   // @see Magento\Customer\Model\Metadata\CustomerMetadata::getCustomAttributesMetadata()
                    'position' => 100
                ]
            );
           $example1Attribute =$customerSetup->getEavConfig()->getAttribute('customer', 'example1');
            $example1Attribut->setData('used_in_forms', ['adminhtml_customer']);
                $example1Attribute->save();
        }    
        $setup->endSetup();
    }
}