When attempting to publish a workflow in O365 you receive the following error:
JWT stands for Json Web Token. To learn more about what a JWT is you can read a short introduction here: JSON Web Token Introduction - jwt.io
The error message states simply that your token has expired due to a long running browser session.
To avoid having to do all your work over again you can simply refresh this token. You can do this by opening a new browser and re-opening the Nintex workflow designer.
This should create a new token and allow you to retry your publish operation.