The Submit button (green with a paper airplane icon) is located in different places depending on whether you're using the web/desktop app or mobile app.
Web and Desktop App
Open a deal. The Submit button appears in the top right action bar.
Mobile App
Open a deal and tap the dropdown arrow next to the deal name at the top. A modal opens with the Submit button (green with paper airplane icon).
Look for the green button with the paper airplane icon—it's the same on both web and mobile.