Hi @Young J (Contractor)@ADM(Mat) DLCSPM@Defence365,
Thank you for posting in this community.
Please follow below steps to troubleshoot the issue.
1.Based on the description of your issue, please first refer this article to check that the Application Discovery and Load Balancer Service is working properly on your farm.
2.Clear SharePoint Designer cache, then republish the workflow.
Note: Microsoft is providing this information as a convenience to you. The sites are not controlled by Microsoft. Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please make sure that you completely understand the risk before retrieving any suggestions from the above link.
3.Create a new 2013 workflow in this list then publish, see if the same error occurs.
4.Create a new 2013 workflow in other list then publish, see if the same error occurs.
5.If the issue persists, you could go to workflow management logs to see whether there are some error messages.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.