First, please install the driver for the iTwo called Universal Control: https://www.presonus.com/products/AudioBox-iTwo-Studio/downloads
Please connect your guitar to the 1/4" input and make sure the button with the guitar icon next to it is set to it's off position (no light). The "guitar button" activates a preamp for passive guitars. When your guitar already has a preamp, you do not need the itwo preamp. In short: The button switches the 1/4" connection between instrument level and line level.
If the issue remains, please create a ticket to technical support.
Here is a link to create a ticket: http://support.presonus.com/hc/en-us/requests/new