After setting everything up as per this guide:
I have added the code.
Experiment Code Validation
No experiment code found.
Google Analytics code found
I am using Optimisepress theme,
Yoast SEO plugin,
Yoast Google Analytics Plugin.
When I manually edit the header.php file to add in the code i get the errors:
Experiment Code ValidationRevalidate
Experiment code found after the Google Analytics tracking code:line:34, column:57,line:313, column:1.
Experiment code found after the Google Analytics tracking code:line:33, column:57,line:345, column:1.
Note: If you're sure that the pages are properly set up, you can still start the experiment.