This rare problem usually appears when starting Excel, and less frequently when saving a workbook. It usually is a sign that something is wrong with your Excel installation. The error can occur in Windows and Mac versions of Excel, but the problem is more often discussed related to Mac Excel.
On the Mac, this error usually means that VBA was not included in the Office installation. This can happen if Office 2011 was installed as an upgrade to Office 2008 (which did not have VBA), not as a fresh installation.
The most common advice is to completely remove Office, including any files and settings that are left behind, then reinstalling. For Mac Excel, the Microsoft Knowledge Base has an article that explains How to completely remove Office for Mac 2011.
This seems somewhat harsh. Before completely rebuilding Mac Office, try this:
Save your work, close Excel, then reboot the Mac.
Follow steps 1 and 3 from the Microsoft article above, then reboot the Mac.
If that doesn’t work, follow steps 1, 3, 5, and 7 in the Microsoft article, and reboot.
If there is still no joy you may have to perform the entire Office transplant.