Magento – Magento 2: new attribute not showing in list configurable product

You need to assign your new attributes to the attribute group. Go to store->products(Under Attribute label). Then drag your new attribute to the attribute group.

For more details about creating attribute check this link https://www.fastcomet.com/tutorials/magento2/product-attributes

There are several other questions related to this problem, and I have not seen this mentioned in any of them.

In addition to the requirements posted by others (attribute should be global, user_defined, visible, and have input type select - or another single selection type) , as of version 2.2.5 there is also requirement that in order to be available for use in configurations an attribute must either have no apply_to (Product Types) specified, or the apply_to must include all three of the following product types, simple,virtual,and configurable.

It is not clear to me if this is a bug or a feature. Here is the applicable code from the file vendor\magento\module-configurable-product\Model\ConfigurableAttributeHandler.php

public function isAttributeApplicable($attribute)
{
    $types = [
        \Magento\Catalog\Model\Product\Type::TYPE_SIMPLE,
        \Magento\Catalog\Model\Product\Type::TYPE_VIRTUAL,
        \Magento\ConfigurableProduct\Model\Product\Type\Configurable::TYPE_CODE,
    ];
    return !$attribute->getApplyTo() || count(array_diff($types, $attribute->getApplyTo())) === 0;
}