Howdy again Robert.
So I downloaded your file and was able to delete all but 2 Masters.
Currently, your Select Avatar slide uses an almost blank Master called "Clean", and the other two slides use another Master called..... "Clean"
This is a good reason why naming your Master's really helps locate those slides when trying to find the culprit for that 1 slide using the wrong Master!
So, right now you should be able to press F4 to enter Master View.
Then Right Click on any Master file on in the thumbnail view panel (left of the Master Slide in your Storyline interface) - and choose DELETE MASTER.
If that option is greyed out, then you still have a slide using it.
To find the culprit, go back in to the Story view in Storyline (press F4 to close Master View), and in the STORY VIEW, you can easily click each slide, and check its LAYOUT in the "Apply Layout" button at the top.
To make it even quicker and easier, you can attach this APPLY LAYOUT menu button to a keyboard shortcut (eg. ALT+5) and that way, you can really quickly hammer through a whole bunch of slides quick.
....ask me how I know ;)
