When trying to adjust setting in the Out of Office Assistant, you get the following error message:
The command is not available. See the program documentation about how to use this.
Go to Help > About Microsoft Office Outlook and click on the "Disabled Items..." button. You will see outex.dll listed. Click on it and then click the Enable button. Fixed!
NOTE: For this fix to work, you must be connected to an Exchange server. If you are using Outlook for personal, then this error is expected since the Out of Office Assistant is not usable without Exchange.