Why am I getting the error "Materials may only contain letters, digits, and spaces or underscores"?

Why am I getting the error "Materials may only contain letters, digits, and spaces or underscores"?

This error occurs because special characters are not allowed in the "Materials" field on Etsy. To resolve this, edit the profile for the affected products and ensure that the materials contain only letters, digits, spaces, or underscores. Once updated, publish the product again from the app to reflect the changes on Etsy.