avrdude: ser_open(): can't open device?

Arduino IDE shows an error:
An error occurred while uploading the sketch avrdude: ser_open(): can't open device "\\.\COM7": The system cannot find the file specified.
How can I solve it?
ANSWER

You should do a checklist in order to jump to a conclusion:

If the COM port is available on Arduino IDE:

If the COM port is NOT available on Arduino IDE:

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