The submit button always appears after the last visible field on your form. When this doesn't happen, it usually means there are required fields that need to be answered before finishing your submission.
To help respondents to see how many fields are left before they can submit, you can enable the "X Questions to Go" feature on your form when using "Standard" mode.
To enable this feature:
If a respondent clicks on the "X Questions to Go" message at the bottom of the form they will be sent to the first field they’ve missed (underlined in red). A message will also be shown stating "This question is required".
If the "X Questions to Go" option is not enabled, the "Submit" button will appear immediately at the end of your form. If the user selects the "Submit" button before finishing any required fields, they will again be sent to the first field they need to complete.