Unknown referenced nature: org.springframework.ide.eclipse.core.springnature

This is a left-over from a prior STS brought-in by the .project file due to a known issue.

To fix this, delete the project from your workspace(s) in STS, and remove the project meta-data from the file system:

  rm .project
  rm .classpath
  rm .factorypath
  rm -rf .settings

When you re-import the project, STS 4 will build proper project meta-data.

I saw this problem after installing Eclipse Jee-2019-03 and using it for a workspace created by Eclipse Oxygen.

My answer is similar to Amadán, but I did not see Spring in the right-click dropdown of the project... because I has not installed the Spring tools yet.

I did this: Right-click on the project --> Properties --> Project Natures
click on springnature --> Remove
click Help --> Eclipse Marketplace...
Find: Spring --> click Go
on the Spring Tools 4 - for Spring Boot --> click Install

after that my project worked and I did not have to add springnature to the Project Natures

You might also try to just:

1. remove the Spring nature
   • right click on the project > Spring > Remove Spring Project Nature
2. add the Spring nature again
   • right click on the project > Spring > Add Spring Project Nature

Open the .project file in your source folder and comment/delete this line:

<nature>org.springframework.ide.eclipse.core.springnature</nature>