In the European Union and Switzerland, it is required by local law to display energy efficiency class information with applicable products. Beginning in mid-January 2021, we'll show "Missing potentially required value" warnings when the energy_efficiency_class attribute is missing for products for which you may be required to display energy efficiency class information. If a required attribute is omitted, your product(s) may be legally non-compliant and you, as the merchant, are legally liable.
To resolve this sort of issue, either provide the missing attribute with a valid value or verify that your product is compliant without the attribute.