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cannot be opened because it contains the following features that are not
supported by Excel in the browser:
• Data validation
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as the error says, somewhere in your spreadsheet you're using data validation (so e.g. you verify an input somewhere and restrict it to certain values). Office Web Apps doesn't support this, so in order to open the file you either need to open it in Excel 2010 directly, or you can open it in OWA if you remove the data validation (though you need to edit it in Excel first as well)
How are thing going on your sides? Would you mark Rene’s Post as answered if he addressed the question? If you need further assistance, please let me know. Thanks.