Getting and error “Cannot complete the install because one or more required items could not be found” in oomp installer

Calculating requirements and dependencies.
Cannot complete the request.  Generating details.
ERROR: org.eclipse.pde.core code=0 Problems occurred while resolving the target contents
  at org.eclipse.oomph.util.OomphPlugin.coreException(OomphPlugin.java:280)
  at org.eclipse.oomph.util.pde.TargetPlatformUtil.activateTargetDefinition(TargetPlatformUtil.java:151)
  at org.eclipse.oomph.targlets.internal.core.TargletContainer.forceUpdate(TargletContainer.java:787)
  at org.eclipse.oomph.setup.targlets.impl.TargletTaskImpl$4.run(TargletTaskImpl.java:1167)
  at org.eclipse.oomph.util.pde.TargetPlatformUtil.runWithTargetPlatformService(TargetPlatformUtil.java:120)
  at org.eclipse.oomph.setup.targlets.impl.TargletTaskImpl.perform(TargletTaskImpl.java:1033)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer.doPerformNeededSetupTasks(SetupTaskPerformer.java:3332)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer.access$1(SetupTaskPerformer.java:3275)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer$WorkspaceUtil$1.run(SetupTaskPerformer.java:4502)
  at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2240)
  at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2267)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer$WorkspaceUtil.performNeededSetupTasks(SetupTaskPerformer.java:4496)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer$WorkspaceUtil.access$0(SetupTaskPerformer.java:4494)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer.performNeededSetupTasks(SetupTaskPerformer.java:3266)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer.performTriggeredSetupTasks(SetupTaskPerformer.java:3241)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer.perform(SetupTaskPerformer.java:3141)
  at org.eclipse.oomph.setup.ui.wizards.ProgressPage$9.run(ProgressPage.java:575)
  at org.eclipse.oomph.setup.ui.wizards.ProgressPage$11$1.run(ProgressPage.java:701)
  at org.eclipse.core.internal.jobs.Worker.run(Worker.java:56)
  ERROR: org.eclipse.equinox.p2.director code=10053 Cannot complete the install because one or more required items could not be found.
    ERROR: org.eclipse.equinox.p2.director code=0 Software being installed: artificial_root 1.0.0.v1526038212974
    ERROR: org.eclipse.equinox.p2.director code=0 Missing requirement: Utils classes for JUnit tests 2.1.100.201803121156 (org.eclipse.papyrus.junit.utils 2.1.100.201803121156) requires 'bundle org.eclipse.papyrus.uml.tools [4.0.0,5.0.0)' but it could not be found
    ERROR: org.eclipse.equinox.p2.director code=1 Cannot satisfy dependency:
      ERROR: org.eclipse.equinox.p2.director code=0 From: artificial_root 1.0.0.v1526038212974
      ERROR: org.eclipse.equinox.p2.director code=0 To: org.eclipse.papyrus.junit.utils 0.0.0

Took 66 seconds.
There are failed tasks.

Solution

I tried the setup using your link and got exactly the same stack trace and underlying error as you.

Someone else also encountered that same underlying problem (i.e. "...requires 'bundle org.eclipse.papyrus.uml.tools [4.0.0,5.0.0)' but it could not be found") and raised it on Eclipse's Papyrus-RT forum.

The response to their problem, dated 2/12/18, was:

If you want the developer environment, unfortunately at this moment the Oomph setup model is broken because of some issues related to the target platform and its dependencies to the base Papyrus. We are trying to fix them but we don't know when it will be fixed.

In the meantime, I'm trying to write down instructions on setting up the environment manually, but it is not easy, and it will take some time.

But I'll post a note here when the instructions are available in the wiki.

So it looks like it's not currently possible to perform the Oomph setup using Oxygen. Note that all of the screen shots in the documentation you linked to are from Mars and Neon, using a much earlier release of Java 8 from 2016.