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if you’re using the free version, you aren’t allowed to use it for commercial purposes; but if you upgrade to a paid version, then you are. And I believe, you can also make a private channel so other people won’t be seeing your creations.

Source: https://www.creativindie.com/midjourney-ai-for-book-cover-design-how-is-this-legal/#:~:text=EDIT%3A%20if%20you're%20using,paid%20version%2C%20then%20you%20are.