You must have 1 (7)-7 & 1 (8)-8.7z.001 in the same directory as your .002 file.
You will need a utility that understands split archives. (Windows) or Keka (Mac) are the most reliable tools for this. Right-click on the FIRST file (the one ending in .001 ). Select 7-Zip > Extract Here (or "Extract to..."). 1 (7)-7 & 1 (8)-8.7z.002
This usually means you are missing one of the parts (like the .001 file) or the download was interrupted and the file is corrupted. You must have 1 (7)-7 & 1 (8)-8
Before you try to open anything, ensure that of the archive are in the same folder. 1 (7)-7 & 1 (8)-8.7z.002