Q. 

Why can’t I export a Budget from a prior year?

A.

The budget export is only useful for exporting the budget for the current year as there are several columns that aren't relevant because they are based on calculations using current dates (YTD, CY Rem, CY Proj to name a few).  If you need data for prior years, there are other reports which will show the necessary budget information (such as the Budget Summary).  In order to export a budget it must be approved and in the current year.