There're thousands of add-ons on the Web for the classic FSX and Prepar3D v1 to v3. Take off from anywhere in the world, flying some of the world’s most iconic aircraft to any one of 24,000 destinations.
A 404 error could be a number of reasons. You could have a problem with your server or Rikooo might have been working on theirs . You might have limited connectivity . You could have some malware involved. I recommend running malwarebytes . It is a free download and you get 15 days of free full service. I have been using it for 4 years, and nothing gets in. Once you run the scan, try and download the scenery again.