Your edit button is now probably within a dataview of type 'TraningEvent'. The page that it refers to is of type 'Registration', this does not match hence the error. You could do a couple of things to fix it, like:
Create a microflow that retrieves or creates the 'Registration' object from the 'TrainingEvent' and call the page from the microflow.
As this is not part of the training module to do anything with the edit buttons I am not sure how to answer this question. My best guess is that you could create a Microflow to do this. You will learn how to do this coming up in the future modules. However, so far I have not done anything with the edit or delete buttons and I am at 7.1 in the learning modules.
I'm sure someone more experienced will answer this question better than I can but I would just leave the onclick action at "Do Nothing" for now and continue on with the training modules.
Hope this helps.