Firstly, I would try clearing the cache from the System application or recycling the application pool.
Are there any errors in the Event log? Many times when elements are not appearing in the UI as expected, there are macro errors and re-signing them can help: https://docs.kentico.com/k10/macro-expressions/troubleshooting-macros/working-with-macro-signatures#Workingwithmacrosignatures-Re-signingmacros
If you open the Modules application, does A/B Testing appear in the list of installed modules? It is part of the On-line Marketing functionality, so if that is installed then A/B Testing should be. If it's not installed, you can install it by modifying your installation with the Kentico 11 installer.
Also, edit your site in the Sites application and check the Assigned objects > Modules tab. A/B Testing should appear in this list- if it doesn't, click "Add modules" and select it.