I would assume your full buildfile contains the ‘war’ plugin
This says the the ONE built artifact is a war file and not a jar
According to maven each project can have one and ONLY one artifact (hence your error).
The question is then: what are you trying to accomplish? Do you want BOTH the jar and war on your nexus? Or just the jar? Why would you need a jar built from the same source as a war?
I my mind a war is an “application” a a jar is a “library”. A “library” is typically something you refer to from other libraries or applications and an application is never refered by anything. They are shipped somewhere. If I venture a guess it would seem that you have common code in your war that you want to publish on nexus. I would reccomend that this code is pulled out of the war project and into a separate project. This could be an excellent usecase of a multi-module build.
I would like some more details on your build to better answer the question.