Click "Publication" tab of the manuscript you want to assign a DOI for.

Click "Identifiers" tab. You will see that there is a warning message indicating that it must first be assigned to an Issue.

--- Future Issues ---
Accepted Manuscripts
Vol. 1 No. 4 (2022)
--- Back Issues ---
Vol. 1 No. 3 (2022)
Vol. 1 No. 2 (2022)
Vol. 1 No. 1 (2022)

You should now see the button to "Assign"

Tip! If you're not seeing this button even after assigning, go back to Issue you created and double check that you have provided all the necessary information for the DOI plugin to generate the DOI. 

To schedule the publication now, "Schedule For Publication." 

Click "Schedule For Publication." This can also be done after all the production steps have been completed.

How to assign Default DOIs on OJS 3.3