YARN Timeline Service v2 unable to start
If you go into 'background operations' (the gear icon in the Ambari UI), then go to the Timeline Service V2 start link (you may have to click on your machine that is running the Timeline Service first to get there), you should have links in the upper right that say 'copy' and 'open'. These will hopefully show you error logs in much more detail.
In my case, the Timeline Service V2 couldn't start because there was not enough memory on the system. It was a small VM cluster just for playing around with only 2GB RAM on each machine. I found through the more detailed error logs that it was giving an insufficient memory error, so when I upped the VM memory to 4GB, it was able to run. My best guess is your memory on the main NameNode where your Ambari UI is running isn't enough. It seems like something around 4GB+ is required, depending on how many services you're running on the main NameNode.