"You don't have a SNAPSHOT project in the reactor projects list." when using Jenkins Maven release plugin

You'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.

Jenkins workspace is not cleanup or you have FINAL version inside pom.xml. Jenkins has check-out strategy for cleaning up workspace.

More: Maven release plugin - SNAPSHOT project needed

Check-out Strategy options:

• Emulate clean checkout by first deleting unversioned files/ignored files, as well as files/directories ignored by svn:ignore, then execute svn update.
• Always check out fresh copy
• Use svn update as much possible, with svn revert before update