HOWTO Fix Unable to set the Installed property of the AddIn class error

1. Close all Excel files


2. Open a blank Excel file outside the Engagement.


3. Click the Microsoft Office Button on the top left corner in Excel, click Excel Options, and then click Add-Ins.


4. Click Manage dropdown list, select disabled item, and click Go.


5. Select all items in the disabled dialog box and click Enable.


6. Close Excel Options then try to open Excel files in Engagement again.