Quickbooks already has a company file open

This happens when you exit quickbooks by simply hitting the close button on the top right corner of the window. If you don’t want to face these kinds of errors in the future then check out for the correct way to close the quickbooks session:
In quickbooks menu head over to close all the applications, Like:

Quickbooks windows > Close all.

Now when you see that no company is further open then you can head over and shut down the program.
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Further Reading