If you have altered your Magento store so that the original store (or any store) with the store code of "default" doesn't exit. Magento may present you with the error "default" store doesn't exist.
Or if you which to change the fall back store (this is the store used when the request url can't be matched to any store) to something other than "default"
By default MDOQ assumes that the "default" store exists and is safe to fallback to.
The easiest way to change this is to update your `pub/index.php` to have the following code (right after the "use" statements)
if(isset($_SERVER['MAGE_RUN_CODE']) && $_SERVER['MAGE_RUN_CODE'] == 'default'){
$_SERVER['MAGE_RUN_CODE'] = 'STORE_CODE';
}
(changing "STORE_CODE" with the desired store code).
You can then release this in the normal way.