DIAGNOSIS PIP/PBP (Picture-in-Picture/Picture-by-Picture) has specific requirements. Both input signals must be active and within supported resolutions.
FIX First, ensure both sources are powered on and displaying an image. Then, check the input selection. Press the joystick. Go to Menu > Display > PIP/PBP. Select PIP Size and Position first. Choose Small or Medium. Then, go to PIP/PBP Input. Select your two inputs, like USB-C and HDMI 1.
IF THAT DIDN'T WORK Lower the resolution on one or both inputs. PIP does not work if one source is 4K 60Hz and the other is also a high bandwidth signal. Try setting one computer to 1080p.
HEADS UP PIP/PBP will disable HDR and may limit color depth. It is primarily for productivity, not gaming or color-critical work. The USB hub only works for the input designated as the primary source.