I can't render my project, what are the possible reasons for this?

It's probably something you did in the comp, or the type of footage you used, or a plug-in you installed, or a render setting you modified. There are billions of combinations of things you can do to a comp, and, depending on your system and version of AE there are an awful lot of combinations of things you can do that might cause you problems. Where do you want me to start? Maybe if you showed us or explained your workflow we might have some ideas that could help.