The Frame Ends (Parts D and E in the Assembly Guide) are boxed together with the Center Rails in Jarvis Frame Box 2 and may appear to be one solid piece. If you are indeed missing one of the highlighted pieces, please contact us for a replacement.
If you can't get the Frame Ends apart, try loosening the pre-installed set screws using the 3mm hex key included with your hardware kit.
Once they are sufficiently loosened, the Frame Ends will slide off the Center Rails with ease.
To view the location of the set screws on the Frame Ends, refer to Step 7 in the Jarvis Assembly Guide, highlighted in the image below: