Essbase spreadsheet specific error - see below
Essbase is an multi-dimensional database. That means there are multiple dimensions within the database. Each piece of data in the database is assigned one member from each dimension. When you retrieve data from Essbase within Excel, each data cell knows what member it represents from each dimension.
There are rules for setting up an Essbase spreadsheet to make this possible. In short, you have a cell that can't identify only one member from each dimension in order to display that information for you.
Generally your company would have an experienced Essbase/Excel user who can tutor you on setting up the spreadsheet for your purposes.