Arduino Uno Doesn't show up under Serial Ports, but it does show up in Device Manager
This happens from time to time.

What can you do?

1- Try unplugging the USB and plug again.

If 1 didn't work.

2- Go to task manager> Processes> find the Arduino IDE that is already running> Right click> End Process tree> unplug the USB> run the IDE again.

Works every time for me.