The file you have chosen is not an Accounting Plus company file.

Troubleshooting Alert Messages Overview > Alert messages (in alphabetical order) > The file you have chosen...
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Situation: The file you attempted to open isn't a valid MYOB Accounting Plus company file, and can't be used by the version of MYOB Accounting Plus on your workstation.

Solution: You can't open this file using MYOB Accounting Plus. Click OK to close this message, then attempt to open a valid MYOB Accounting Plus company file.