Hi Ricardo,
This is a known issue in Power Pivot. This issue always appeared when you try to modify a PowerPivot model in Excel while you have an SSAS Tabular instance running on the same machine.
There is a workaround you can try is to disable the SSAS Tabular service and you can edit your PowerPivot model.
For getting more information about this issue you can read this blog:
http://www.mssqlgirl.com/powerpivot-2013-error-data-model-damaged.html
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
If you need more help with Power Pivot please post your issue in Power Pivot forum:
https://social.technet.microsoft.com/Forums/office/en-US/home?forum=sqlkjpowerpivotforexcel&filter=alltypes&sort=lastpostdesc
The reason why we recommend posting appropriately is you will get the most qualified pool of respondents, and other partners who read the forums regularly can either share their knowledge or learn from your interaction with us. Thank you for your understanding.
Hope its helpful.
Regards,
George Zhao
TechNet Community Support