When trying to paste data into Excel, you may run across this error. The most likely cause is that you have selected a group of cells to copy and then when you want to paste, you have selected another group of cells that are not the same cell size. Usually you can get past this error by selecting only a single cell and pasting to that cell. The cell you selected will act as the top-left most cell in the pasted selection, or the top most cell if it's only a single column of data. However, there is another cause for this error that can be elusive.

Check to make sure that you have not grouped your sheets. There are a couple of ways to tell. First, ungrouped sheets are a different color than the selected, active sheet. In the screenshot below, notice the different color between Sheet1 and the other sheets.

In a grouped set of sheets, notice they are all the same color.

One other way to tell is to look to the top of your workbook. If you see [Group] beside the workbook name, then you know you have grouped sheets. You can't copy a single cell from a group of sheets and paste to a workbook cell with non grouped sheets. What Excel is trying to do is copy the cells you selected from not one workbook, but from all of the grouped workbooks, as if they were stacked. Then when you select a single cell in an ungrouped workbook, Excel determines that you are trying to paste the grouped cells into a single cell on a single worksheet and says "Nope, can't fit all of that there!" Imagine trying to fit a single stack of 100 folded towels into a small suitcase and close the lid. It would be impossible.

To group worksheets, select a worksheet and then hold the SHIFT key down and select another worksheet. You can also hold down the CTRL key to do a relative selection (you can pick and choose). When multiple sheets are selected in this manner, the sheets are automatically grouped and the [Group] warning appears at the top of the workbook. You can ungroup the sheets by selecting only one of the worksheets in a grouping and the rest will turn the darker color. You can also right click a group of selected sheets and left click the "Ungroup Sheets" option.