Hi,
When using keyword insertion, the formula and default ad text you specify must be formatted correctly. Please ensure that you place the default ad text within {squiggly brackets}, as shown here: {keyword:default text}, where "default text" is the word or words you want to appear when the text can't be replaced by a keyword. Please retry the request after making this change and let me know if it works.
Regards,
Shwetha, AdWords API Team.