To reduce the line spacing in the legend box in Microsoft Excel, you can follow these steps:

1. Click on the chart to select it.
2. Click on the legend to select it.
3. Right-click on the legend and select "Format Legend" from the drop-down menu.
4. In the "Format Legend" pane that appears on the right-hand side of the screen, click on the "Options" tab.
5. Under "Text Layout", you will see options for "Line Spacing" and "Text Direction". Adjust the "Line Spacing" to your desired value.
6. Click "Close" to apply the changes.

If you have already tried this and it did not work, you can try adjusting the font size of the legend text. Sometimes, a larger font size can cause the line spacing to appear larger than it actually is. You can also try resizing the legend box itself to see if that helps. If none of these solutions work, it may be a glitch in the program and you may need to recreate the chart from scratch.