I think it means that the body has already been read by using either .json() .text() etc… When you run x.json() it takes the response’s body and reads it into JSON. If you try to run x.json() again it will give you that error. So you could only use one of these methods once. So I am assuming somewhere in your code it is reading the body of the same response again using one of the Body methods.
I think that is why they offer the Body.bodyUsed method. So you can see if it has been read already.