Getting 'Invalid API key or exceeded limits' exception

I am trying to access the Virus Scan API to scan pdf files for malware on upload however, I am getting the exception mentioned above whenever I upload a file and I can't figure out why that is happening. I followed the instructions on Cloudmersive's documentation page, generated an API key and set it as required. What could be the issue?

PS: I am uploading a pretty small file (8MB)