Your pictures were overexposed.
The Instax Mini 8 does not set exposure automatically, although it does suggest the correct setting.
Before you take a photo, point the camera in the direction of your subject. A dial around the lens is used to set the correct exposure. There are 4 indicators next to the dial, one of these will light up, suggesting the correct setting. Turn the dial so that the mark corresponds to the indicator light, and take the photo.
Also, there are two little round windows next to the flash. These are the exposure sensors. Be careful not to cover these with your fingers when you hold the camera, because if you do, the camera cannot measure the light and/or control the power of the flash correctly, which will cause your pictures to come out overexposed. In fact, this might be exactly what happened.