Solution
This error means you're signed in to your MYOB account but you don't have access to the file you're trying to open. This can happen if multiple company files have been opened on the same computer and the files are listed in the Recently opened company files list on the AccountRight Welcome window.
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If you try to open one of these files but you're not entitled to do so, the error will display. So make sure you're only opening company files you're entitled to access, or request that access from the company file owner so they can add you as a user.