Error (169079): Pad of non-differential I/O pin in is too close to pad of differential I/O pin in – pads must be separated by a minimum of pads. Use the Pad View of Pin Planner to debug

You may get this error message in the Quartus® II software when assigning an I/O pin with a non-differential I/O standard assignment into an I/O location that is  too close to the specified differential pin. Differential and non-differential I/O pins must be separated by a specified amount of device I/O resources.

Use the Pad View window of the Pin Planner in the Quartus II software to check the pad placement, and then assign one of the conflicting I/O pins to a different location or just complete the IO standard assignments and compile to allow the fitter to automatically assign a correct pin placement for your design.