DIAGNOSIS: You're likely plugged into the wrong HDMI port, or your HDMI cable isn't high-bandwidth enough. The U7K has four HDMI ports, but only two (HDMI 3 and 4) are full-bandwidth HDMI 2.1 ports that support 4K at 120Hz.
FIX: 1) Move your PS5 cable to HDMI port 3 or 4 on the TV. 2) On the TV, go to Settings > Inputs > HDMI Signal Format. Select the port you're using (HDMI 3 or 4) and set it to 'Enhanced (4K 120Hz)'. 3) Ensure you're using the HDMI cable that came with your PS5, or a certified Ultra High Speed HDMI cable. Older 'High Speed' cables won't cut it.
IF THAT DIDN'T WORK: On your PS5, go to Settings > Screen and Video > Video Output. Check 'Enable 120Hz Output' is on. Also, try manually setting the 4K Video Transfer Rate to '-1' or '-2'. Sometimes the automatic setting gets confused with certain TVs.
HEADS UP: When you enable 4K 120Hz, the TV might disable some other picture features like certain local dimming zones or color enhancements. This is normal. The processor can't do everything at once at that high bandwidth.