When I execute the command
gcloud compute copy-files \"C:\\Users\\fName lName\\Desktop\\testtext.txt\" instancename:test.txt --zone europe-west1-a
In order to copy the file testtext.txt that you are specifying, you need to be in the path where that file is and specify its name while copying not the path.
Example: from your command line lets suppose you are in this path:
C:\Users\fName lName\Desktop\
Your command should be the following:
gcloud compute copy-files --zone europe-west1-b testtext.txt instancename:/PATH_where_you_want_the_file