The Arduino IDE can’t find my Arduino Uno
It can have many reasons, but you can try the following:

First time use?


Check if you need to install a special driver for CH340


Environment/Software problems


Try another USB port
Change USB cable
Restart Arduino IDE
Restart computer
Reinstall Arduino IDE


Hardware problems


Test using another Arduino
Test using another computer/laptop
Replace USB cable
The Arduino is (partly) broken, nothing much to do here