Confluence doesn't have "folders", it's made up of spaces and pages within them. I'm going to guess you mean a page.
Could you explain exactly what you are trying to share, how, and give us the full text of the error message?
But before that, please check that the intended co-worker has an account with the permissions to see the page(s) you're trying to share - check the space permissions let them in, and that the page does not have restrictions that block them.
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