It seems to be that this testing option is only availabe using single email deliveries. In my company we use many recurring email sendings, so that's a future wish list item.
One of our Adobe contacts will check if this conclusion is right.
And to see the 'prepare option' I've had to check the box by 'request confirmation' in the schedule part of the email delivery.
So it's a bit of a workaround now 😊
- save the current recurring email as a template
- create a new single email delivery in the existing workflow
- base the content of this new email on the saved template in step 1
- check the box 'request confirmation' (otherwise the email will be sent right away)
- add the profile substitution
- get your email test and check it (don’t hit the ‘confirm’ button before testing)