🔧 Amazon - Error Invalid value for apparel size attribute

Amazon error message (example):

Based on the data from [shirt_size#?.size_system, age_range_description.value, target_gender.value, shirt_size#?.size_class], the value “6_years” for the “size” field in the Clothing Size attribute is invalid. Please provide a valid value. (Attributes: shirt_size)

What does this error mean

Amazon detected an invalid size value for the product. The value entered here, (“6_years”), does not match Amazon’s accepted size values for this category. 
If your size value does not match Amazon’s accepted values for these attributes, it will be rejected.

How to fix this error

Provide a valid size value from Amazon’s accepted list for this category. Use Amazon’s official size charts to select the correct size.

Example here: Instead of 6_years, use the exact value Amazon expects for child apparel (e.g., 6Y, 6 Years).