PowerPivot performing another operation (and not allowing you to do anything else)
One of the more frequent error messages we get when using PowerPivot is ‘PowerPivot performing another operation’ and as a result you cannot create a calculated field or import data or do anything with PowerPivot.
Related: Learn how to use PowerPivot
Sometimes this is normal and if you wait a few moments it allows you to continue. However, other times it seems to be stuck in this mode. The easiest way to address this is to close the PowerPivot window (save if you need to). When you reopen the PowerPivot window it should be ready to operate.
Occasionally this also does not work. In this case you need to close the Excel window and re open.