Steps to reproduce:
1) Fresh install of Jenkins
2) Skip the "install suggested plugins"
3) Install only Robot Framework plugin + restart
4) Create a new freestyle-job
-> "Add post-build action" does not show "Publish Robot Framework test results"

Workaround:
At step 2), don't skip, but instead choose to install the suggested plugins
-> now in a new job you'll immediately see the choice "Publish Robot Framework test results"