From the Premiere Elements Information FAQ http://forums.adobe.com/thread/1042180
•What version of Premiere Elements? Include the minor version number (e.g., Premiere Elements 12 with the 12.0.1 update).
•What operating system? This should include specific minor version numbers, like "Mac OSX v10.6.8"---not just "Mac"
•Has this ever worked before? If so, do you recall any changes you made to Premiere Elements, such as adding Plug-ins, brushes, etc.? Did you make any changes to your system, such as updating hardware, printers or drivers; or installing/uninstalling any programs?
•Are you using an account with Administrator Privileges?
Run as Administrator http://forums.adobe.com/thread/969395 (Encore + "All" Premiere)
•Have you installed any recent program or OS updates? (If not, you should. They fix a lot of problems.)
•What kind(s) of image file(s)? When talking about camera raw files, include the model of camera.
•If you are getting error message(s), what is the full text of the error message(s)?
•What were you doing when the problem occurred?
•What other software are you running?
•Tell us about your computer hardware. How much RAM is installed? How much free space is on your system (C:) drive?
And some other questions...
•What are you editing, and does your video have a red line over it BEFORE you do any work?
•Which version of Quicktime do you have installed?
What is your exact brand/model graphics adapter (ATI or nVidia or ???)
What is your exact graphics adapter driver version?
Have you gone to the vendor web site to check for a newer driver?
For Windows, do NOT rely on Windows Update to have current driver information
-you need to go direct to the vendor web site and check updates for yourself
ATI Driver Autodetect http://support.amd.com/en-us/download/auto-detect-tool
nVidia Driver Downloads http://www.nvidia.com/Download/index.aspx?lang=en-us
IF you have version 10 and nVidia
nVidia Driver Rollback PreElements-10 http://forums.adobe.com/thread/1317675
-Old Drivers http://www.nvidia.com/Download/Find.aspx?lang=en-us
Let us get to specifics.
Based on your earlier threads, you appear to be working with Premiere Elements 12 Mac. Is that correct?
Where do you have the Scratch Disks directed and how much free hard drive space is at that location?
See Edit Menu/Preferences/Scratch Discs and the Media Cache choice.
What specifically does your error message say when you attempt to render the Timeline content?
Are you working with your computer with the Internet connection on?
Please delete the Adobe Premiere Elements Prefs file to determine if that has any effect on the error.
See Library/Users/User Name/Application Support/Adobe/Premiere Elements/12.0/
And, then the background questions about your project...
a. Project Preset
b. Properties of Your Source Media - also is the "do not render" for all Timeline content of just one particular format?
c. If you start a new project, do you have the same problems.
Let us start here and then we will let your answers help us to customize a Premiere Elements answer for your Premiere Elements question.