I've been experimenting with Google Optimize and have tripped up a couple of times. First I made the assumption that Google Tag Manager would be the best way to implement the experiments. Second I didn't appreciate the linkages needed through to Google Analytics required to make Optimize work.
Don't Implement Using Google Tag Manager
Set Up Your Goals in Google Analytics First
Not being a heavy Google Analytics, 'GA', user I didn't realise the extent that Optimize depended on GA. Optimize really only implements the page variant delivery part of the testing. All the heavy lifting for the analysis of the results is run in GA. Before you try to set up your experiment in Optimize you should set up your goals in GA. See here for how to set up goals. Once you have your goals in GA then you will be able to link your experiment to the achievement of the goals.