This message 'Cannot resolve org.jenkins-ci.main" is a known IntelliJ-Maven integration problem (see https://groups.google.com/g/jenkinsci-dev/c/RGlvJSxq8gc/m/OU-Ts_zsEAAJ). However, it does not hurt, everything else works as it should.
BTW: I do not get those error messages anymore with IntelliJ 2021.3 and Jenkins 2.289.1.