In general, AI is not very good in understand negation in natural language sentences, likewise the Generation Frame Prompt is also understood as positive sentences: whatever is mentioned there, AI will try to make it show up in the image.
That’s why asking it for “a cucumber sandwich without crust” will not work, the crust will be included. You need then to use the Negative Prompt feature.
To display the Negative Prompt, click on the regular prompt and the on the “+ Neg” button, the Negative Prompt will show up:
You can now write which elements are not expected to be seen on the picture, and then you can finally get your crustless sandwich: