After the expense is created, and you have filled in all details, you can click on the green button 'Save and submit' to let the expense continue the approval flow.
If the button 'Save and submit' is unclickable, it means you did not fill in all of the required fields.
These empty but required fields will be indicated in the app and online. If you do not have all of the information yet, you can still save the details you already filled in by clicking on the 'Save'-button. Once the required fields are filled in as well, you will be able to click on 'Save and submit'.