Magento 2.4.6: $_isFixed must not be defined error in custom shipping module

I have coded a custom shipping module for Magento 2.4.6 according to Adobe’s documentation here: https://developer.adobe.com/commerce/php/tutorials/frontend/custom-checkout/add-shipping-carrier/

I am receiving the following error upon running a static content deployment:

Fatal error: Type of MyModulesCustomShipping1ModelCarrierCustomshipping::$_isFixed must not be defined (as in class MagentoShippingModelCarrierAbstractCarrier) in /var/www/html/app/code/MyModules/CustomShipping1/Model/Carrier/Customshipping.php on line 18

My code for the Customshipping.php is below:

declare(strict_types=1);

namespace MyModulesCustomShipping1ModelCarrier;

use MagentoFrameworkAppConfigScopeConfigInterface;
use MagentoQuoteModelQuoteAddressRateRequest;
use MagentoQuoteModelQuoteAddressRateResultMethod;
use MagentoQuoteModelQuoteAddressRateResultMethodFactory;
use MagentoQuoteModelQuoteAddressRateResultErrorFactory;
use MagentoShippingModelCarrierAbstractCarrier;
use MagentoShippingModelCarrierCarrierInterface;
use MagentoShippingModelRateResult;
use MagentoShippingModelRateResultFactory;
use PsrLogLoggerInterface;

class Customshipping extends AbstractCarrier implements CarrierInterface
{
    protected $_code = 'customshipping';

    protected bool $_isFixed = true;

    private ResultFactory $rateResultFactory;

    private MethodFactory $rateMethodFactory;

    public function __construct(
        ScopeConfigInterface $scopeConfig,
        ErrorFactory $rateErrorFactory,
        LoggerInterface $logger,
        ResultFactory $rateResultFactory,
        MethodFactory $rateMethodFactory,
        array $data = []
    ) {
        parent::__construct($scopeConfig, $rateErrorFactory, $logger, $data);

        $this->rateResultFactory = $rateResultFactory;
        $this->rateMethodFactory = $rateMethodFactory;
    }

    /**
     * Custom Shipping Rates Collector
     *
     * @param RateRequest $request
     * @return MagentoShippingModelRateResult|bool
     */
    public function collectRates(RateRequest $request)
    {
        if (!$this->getConfigFlag('active')) {
            return false;
        }

        /** @var Method $method */
        $method = $this->rateMethodFactory->create();

        $method->setCarrier($this->_code);
        $method->setCarrierTitle($this->getConfigData('title'));

        $method->setMethod($this->_code);
        $method->setMethodTitle($this->getConfigData('name'));

        $shippingCost = (float) $this->getConfigData('shipping_cost');
        $method->setPrice($shippingCost);
        $method->setCost($shippingCost);

        /** @var Result $result */
        $result = $this->rateResultFactory->create();
        $result->append($method);

        return $result;
    }

    public function getAllowedMethods(): array
    {
        return [$this->_code => $this->getConfigData('name')];
    }
}

I had changed protected string $_code = ‘customshipping’; to the current iteration shown above to try and solve the issue but that did not work. Any help would be greatly appreciated.