There is a user on the Steam forums that has laid out a solution to this problem. It has worked for other users in the topic. It also involves a bit of meddling with the registry, be careful to only change keys listed in this post. The original post is a bit clustered; the steps are laid out below:
- Open your start menu, type regedit and press Enter.
a. (For 64-bit versions of Windows) Navigate to key HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432NODE\Sims(Steam) by using the drop arrows next to the folders.
b. (for 32-bit versions of Windows) Navigate to key HKEY_LOCAL_MACHINE\SOFTWARE\Sims(Steam) by using the drop arrows next to the folders.
Right click on the folder for Sims(Steam) and select Export.
Save the file to your Desktop as whatever you choose, make sure that it is saving as type: Win9x\NT4 Registration Files, it can be selected in the drop down under the box for inputting the name of the file.
Right Click the file saved to your Desktop and select Edit.
Replace all instances of Sims(Steam) with Sims, then save your changes.
Go back to the Registry Editor, then open the File menu and select Import.
Find the file that you just edited and click Open.
Reboot, your Create-A-World should now work.