I have two Arduino Uno's. One of them works fine, and shows up as "com3" under Serial Ports. The other, however, tries to work as "com6", but I keep getting the error "Serial port com6 not found." I have tried restarting the program with the Arduino plugged in (which was suggested for similar problems), restarting the computer, and hitting the Reset button on the Arduino. Also, the Arduino that works shows up under the Device Manager, and the other one doesn't.
The Arduino software has no idea what which COM ports does what. So something else might be creating "COM6" and the IDE picks it up as the only serial port.
You need to solve the problem of why your board isn't showing up in Device Manager.