He did because you have an error and we are trying to help you figure out what you do wrong. In this case the error seems to be misleading as it says to specify only 1 connection string. I do not see any on your command. And according to https://docs.mongodb.com/database-tools/mongoexport/ you do not need to specify any when connecting to localhost at the default port. By looking in more details, I see that your --fieldFile argument has a space in it. You should put it in double quotes.
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