Here are the steps on How to enable maintenance mode in Bigcommerce:
1. Go to Store Setup › Store Settings, then toggle the Store Status setting to Down for Maintenance.
2. Enter a custom message to be displayed to your customers or leave the default message. Although there is no character limit on this field, the page on your storefront which displays it was not designed to handle more than ~2,000 characters.
3. Save your changes.
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