Right, the problem is you have activecell on both sides of the equal sign. The ActiveCell property only identifies the active cell on the active sheet. So if CoA is not the active sheet, Sheets("CoA").ActiveCell doesn't make any sense.
Can you use the cell index instead? For example:

ActiveCell.Value = Sheets("CoA").Cells(1,1).Value ' But your cell instead of 1,1.