Ensure you have GreenOlives6.7z.001 in the same folder. If there are more parts (e.g., .003, .004), you must have those as well.
The file is the second part of a split compressed archive. Because it is a split file, you cannot open it by itself; you must have the first part (and all subsequent parts) in the same location to extract the data. Prerequisites GreenOlives6.7z.002
Hover over 7-Zip and select "Extract Here" or "Extract to GreenOlives6/" . Ensure you have GreenOlives6