Ok, you’re close.
That message just means you do not have one of the supported Rom files.
Not sure which one you have. Try using a different Rom file if you can.
Don’t give up!
Are you using Mac or Linux?
You need your Rom file and the starforc_nex.zip file from here in the same folder - this will usually be your Downloads folder on your machine.
Then you should be able to follow the appropriate section of the video guide here: https://youtu.be/bfyol449GFY?si=-wBsZQV4TbxifBGb
I have tested the download with AVG on MacOS and although I was able to download the file, I have reproduced your issue once I unzipped the downloaded file.
I have reported the utility to AVG as a False Positive. I am awaiting their response.
If you would like to use the file, you can do so by following this process.