I don't know what's wrong with my Terraria world transfer?

If you never managed to solve your problem,

assuming that you didn't delete your old files and you've got the game through Steam

I recommend that you use the in-game cloud saving system for transferring, this would require rebooting your old PC, at the bottom left of each slot for both characters and worlds there should be 3 icons. Click the last one that appears to be a cloud and you've moved it to the cloud, Steam should automatically download these files, after starting Terraria you'll probably get a notification asking if you want to use your local files or download from the cloud since you've modified the save files, pick the option to get the clouds versions and it should download your saves straight to your PC.

FYI- there are two separate save locations for Terraria one is by default Documents\my games\Terraria (Windows) and one in the Steam\Userdata, the user data information is for storing cloud data and documents is for local data.

If your data loads successfully and you can play them on your new computer click the cloud icon again to take them off the cloud and store them locally or you can keep them in the cloud and locally store their .bak files as a backup in case anything goes wrong.

If you solved this yourself I hope this helped someone.