Go to Workflow Admin –> Settings. Make sure that “Activate Workflow process ?” is checked.
If it’s checked, then this is most likely a permission issue. Check if the given role has “Submit to Workflow” and/or “Sign off” custom capabilities. Check out “custom capabilities”
Another issue could be related to Javascript errors preventing the display of “Submit to Workflow” button. Check your Javascript console for an errors.
One such error was caused due to Javascript error in Yoast SEO 3.3.0 plugin – Check this link for more information – https://wordpress.org/support/topic/javascript-errors-causing-problems?replies=2