It's right there in the first action on your screenshot - "offset 0,0 size 0,0"
You need to specify the correct size and offset if you want to take a snapshot of a portion of the screen. Be aware that these values are in device pixels. You'll need to use CanvasDeviceWidth and CanvasDeviceHeight expressions of Platform Info object to convert to device pixels.
Example:
| Global number r = 0
+ (no conditions)
-> System: Set r to PlatformInfo.CanvasDeviceWidth÷ViewportWidth(0)
-> System: Take snapshot of canvas (PNG, quality 75, offset 100×r, 100×r size 500×r x 300×r)