If the user attempts to upload one of their maps to Steam Workshop, but Steam needs them to agree to the terms and conditions of Steam Workshop before doing so, then this error will show up. I don't know if 'Steam Workshop Legal Agreement' should be localised or not. The link to read and agree is https://steamcommunity.com/workshop/workshoplegalagreement/. In some cases it is called the Steam Subscriber Agreement.
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Click here to view and accept the agreement.