What does "Too many attribute values for attribute: size (color)" Google error mean?

This issue can occur when an item contains alternative sizes or colors even though it should represent a single variant.

Under Feed data options, choose to export each combination as an individual item for products with size or color variants. Then check color and size mappings.

Inspect two XML items: they should have different IDs and each should contain its own values. A genuine composite size is not a list of alternative sizes. Likewise, an actually multicolored variant can describe its colors in Google's accepted format; it should not enumerate every color available in other variants.

A product without variants does not need artificial combinations.

For detailed requirements: Official Google documentation (1), Official Google documentation (2).