Question: When I try to render, I get an error that says there are missing source files.
Sometimes After Effects acts finicky with files that you import into the program. Let's use one of the files on this site as an example. The "Metal Movie Trailer Text" project file.
In your project window toggle down the "stills" folder. Right click the "Metal Texture.psd" file and select "Replace Footage" then click "File".
Browse to the folder that this After Effects Project is located in that you downloaded from the website. Click on footage > Stills > then click on "Metal Texture.psd"
You'll see that it shows up as a connected file in the project window now.
Do this for all the files that After Effects claims are missing.