''If you are unable to get your serial to work properly there is a few different possible causes you need to figure out.''

Make sure your serial is for the correct domain name

Each serial is valid for a specific domain name. When you asked for a serial, you gave a domain name along with the request. The serial you have received is only valid for that single domain name. So if you are trying to use OneStepCheckout on a different domain, you will get a serial error.

Check your error message, it displays the domain name

Serial validation uses the domain name that web server returns. Web server may be configured incorrectly and incorrect domain name is used for serial validation. If error message displays incorrect domain name in "Please enter valid serial for domain YOUR DOMAIN", you need to correct your web server configuration

Check if you are entering serial to correct store view

In Magento admin top left there is a dropdown for store views. If you have several store views, make sure you are entering your serial for the correct store view, select the correct store view and then enter your serial for that domain name.

Make sure the cache is disabled

After entering a serial you might need to refresh your cache under '''System -> Cache Management'''. Please refresh or disable the cache and try the frontend again.

Make sure you don't have any unwanted characters

Sometimes when you copy and paste text you get an unwanted whitespace before or after the serial. Please double check the serial that there are no whitespaces before and after the actual serial.

'''Since OneStepCheckout version 3, all serials became subdomain-independent. So if you have a serial for "domain.com", it will work for "store.domain.com", "something.store.domain.com" and so forth.'''

Check what domain your website is configured for

If none of the above solutions work for you, there could be a different issue. Sometimes webservers are configured to display a different domain name than the one you are actually accessing through your browser. The PHP variable <?php echo 'Domain: ' . $_SERVER['SERVER_NAME']; ?>
is used to generate the serial.

Please create a PHP file and upload it to your Magento installation with the following contents:

<?php echo 'Domain: ' . $_SERVER['SERVER_NAME']; ?>

Then run the file in a browser, and it will display the domain name OneStepCheckout is validating your serial against. If it is different than the domain you have specified, you need to contact us and inform us about the correct domain name so we can provide you with a new serial.