MobileTrans will always prompt to enable USB debugging on the phone if it's turned off. Have a double check on the device and ensure you turn on USB debugging.
If you have already ticked the USB debugging box, there is another way to fix it:
1. Scroll down the phone screen, and tap on Charging via USB to see more options. If you do not see it, re-connect the phone to the PC via a USB cable. Once the connection is enabled, choose the USB connection mode from the prompt.
2. Select either USB mode among the three options, and look at MobileTrans software once you select one mode. The three options for USB mode may be shown differently because of Android models. Do remember to switch smoothly. MobileTrans software will respond every time you switch to a different USB mode. The MobileTrans interface changes to the normal connection page if the USB mode works.