I'm trying to download this;

https://rctgo.com/downloads/view/coal-canyon.7441

However, when I press the download link, it shows this error...

<?xml version="1.0" encoding="ISO-8859-1"?>
<Error>

Code:

<Code>AccessDenied</Code>

<Message>Access Denied</Message><RequestId>43C1AD98057B6D53</RequestId><HostId>jZX489niSfiZrmL0a2CxVrNK+nV5snTS+jdIhgGwDvAY0ea0W+1y5FM1m358PEeds68Nplov+mM=</HostId></Error>

Why is it doing this, and how can I fix it?

Good catch. I haven't seen that error before, but it appears that the file is actually missing from the download. As in, the "File" box is inexplicably blank.
I don't know if this means the download is gone from the entire system or just from that page, but if it's the latter, there could be a way of salvaging it by entering the information manually to potentially match it up with the original file information (there's a similar, hidden version of this file, as it so happens, uploaded a few days earlier). Justin will have to confirm whether or not this is actually possible since he's the one who knows how these internal systems work.

Long story short, you should be able to download it now.

The file wasn't on the server, but as Terry said the creator had apparently uploaded the same file not long before entering it into the Build It competition. That download was hidden from view but the file for that download was on the server.