First, make sure you don't already have an ESP8266 core version installed using the Board Manager (see above). If you do, uninstall it from the Board Manager before proceeding. It is also advisable to ...

It might happen; you get “Error opening serial port” while trying to upload a sketch after you have selected your board and serial port. It happens due to ...

So far, AFAIK the only official ways to program Arduino boards were through the Arduino IDE program, or the cloud-based Arduino Create which works in your web browser ...