Open the note type, look what the field is called that contains the image and set this field‘s name in your card template instead of {{Sentence-Image}}.
So if the field has the name image set {{Sentence-Image}} to {{image}}.
It sounds like you’ve edited your note type following some guide – or imported a broken note type. At a high level – in order to use a field on your template, that field has to exist in your note type.
Okay, I see your screenshot. What you’re looking at right there is the back template for Card 1 “Recognition” in the note type “Core 2000.”