Error opening serial port


I've a problem with my arduino adafruit-feather-m0-adalogger. I'm trying to work with this libary GitHub - sparkfun/SparkFun_ADXL345_Arduino_Library: Arduino Library for the ADXL345 and trying to run this example code SparkFun_ADXL345_Example.

First: I load the Blink example
--> Everything works - led is blinking
(Port is still selectable)

Second: I load the SparkFun_ADXL345_Example, upload it on the arduino
--> "Done uploading"
(Tools>Port: Port is now greyed out)

Third: I load the SparkFun_ADXL345_Example, change it slightly - uncomment the Serial.print part

  • Try to upload it - Port is closed (Can't select a port, because it's still greyed out from the second step) - Do a double click on the reset button on the arduino board - Port is now selectable -Select Port - Upload sketch
    --> "Error opening serial port COM4"

What did I do wrong?

Thanks in advance!