If you have renamed or deleted some objects in the scene that our setup script requires then this can happen, therefore we recommend re-downloading the original zip file and starting from scratch. Please use our Discord channel and we will be happy to try and help you. https://discord.com/channels/1201528414444736512/1201528416202133519 

All of our scenes have been designed to work using the Cycles Render engine, although some work fine with EEVEE we recommend using Cycles for best results

The short answer is yes, all of our scenes can be edited however you like, but be warned that our "Scene Setup" panels are configured to work with the existing scene contents and any objects you add to the scene might need additional "Collision" or Rigid body setups configuring to work with the scene.