不能赋值:“i”在swift中是一个“让”常量

So basically i am trying to assign 2 random numbers up to 20 in 2 labels and the user will have to find the correct result. A different view will appear based on if the answer is correct or not and this will happen 10 times. The problem is that I get an error on the counter "i" that I use and even though I declare it as variable, I get an error saying that it is a constant.So basically i am trying to assign 2 random num