If you look inside the core-18.0.01 folder you have, you should see the contents are very similar to the root of your Gibbon install. The contents of the core-18.0.01 folder, not the folder itself, need to be copied into your Gibbon root, which will overwrite files in your root with the updated files for v18.0.01
Thank you so much, all sorted for the moment!
Great to hear! If you haven’t already done so, you can visit the Update page and run the database updates for v18.0.01