在字符串中从parse.com保存数据的问题

I want to get a record from my parse.com class called "Tags" by ID. I want to retrieve data from the object "username" and "tagtext". Afterwords I want to save "username" to the String "username" and "tagtext" to the String "tagtext". My code looks like this, but I'm getting an error called I want to get a record from my parse.com class