I came across this question when working on an open source project I don't own where I wanted to submit an update to a site being generated in this manner. I wanted to test and view the change locally, but kept hitting this error.
User @Frischling alluded to this above (credit to them). It turns out what I wanted was not to edit any existing information or update any pom.xml files, but just build entirely in offline mode.
I was trying to run this command and it was failing with the error the original poster mentioned:
mvn site
To do this build offline instead, execute the following commands:
# Download the dependencies for the target
mvn dependency:go-offline site
# Build the target offline
mvn --offline site
Then the output got correctly generated to the target/site
directory like I expected.
It's not ideal if you own the project or part of the project, but for a case like mine where I owned none of it, it was the perfect option.