So this code still isn't working and I can't understand why? I've added the comma and it's made no difference, I've also removed the other fields thinking they may have caused an issue.
What type of property is the nps_flag property? When updating enumeration or single checkbox contact properties, your value must exactly match the internal property values. If nps_flag is a single checkbox, the internal value for true is all lowercase (i.e. true). Can you try your request again with the lowercase property value?