Rebillia creates Product Modifier Options when setting up a subscription. The modifier options are added onto order line-item options, which makes the order to appear as missing variant.
To resolve this, navigate to the Customizations section of the product page on BigCommerce, and copy the "option name" found in the "Modifier Options" section.
Then, visit your store's settings, and scroll to the Access credentials & integration settings panel, and enter the "option name" from BigCommerce (e.g. "Subscribe and Save 25% On Every Auto Ship Order") under the "Customization Option Names" section.
Note: Provide the "option name" of the modifier options, NOT the actual "values" of the modifier option (e.g. "medium").