DIAGNOSIS The USB-C port on the PA278QV is for video, data, and power delivery (up to 65W), but it needs to be the only connection and your MacBook might need a nudge to recognize it as a power source.
FIX First, use the USB-C cable that came with the monitor or a certified high-quality Thunderbolt 3/4 cable that supports DisplayPort Alt Mode and 65W+ PD. 1) Connect the cable to your powered-on monitor first. 2) Then connect it to your MacBook. The monitor should wake up and switch to the USB-C input. If it doesn't, use the monitor's joystick to manually select 'USB-C' as the input source. For charging, make sure your MacBook is awake and not in sleep mode when you connect it. Sometimes you need to unlock the screen for the handshake to complete.
IF THAT DIDN'T WORK Your MacBook's power management might be interfering. Go to Apple Menu > System Settings > Battery. Check that settings aren't preventing charging. Also, try an SMC reset on your Intel Mac, or a shut down and restart on Apple Silicon Macs. As a test, connect the monitor to AC power with a different device, like a modern iPad, to verify the USB-C port is working.
HEADS UP The 65W charging is enough for most 13-inch and some 15-inch MacBooks under load, but larger 16-inch Pros may still slowly drain under heavy use. That's normal. For a guaranteed charge, keep your MacBook's original charger handy.