The error message is pretty self explanatory in my opinion: You'reYou're trying to release an artifact that's not a snapshot. That means your artifact's version number is something like 3.0.3. That version number implies its already been released. You can't release a release. There would be no changes in between and therefore no point.
You're only supposed to release SNAPSHOT versions. That means your version number would be like 3.0.3-SNAPSHOT.