What’s the Issue?
The out of the box approval flow template for SharePoint is missing a vital part of the process to get the File Metadata - ETag and then setting it when the item is approved. ETag is a special identifier for the Item version.
Solution
The article below is an excellent solution to the problem experienced by one of my clients, hopefully it can help other is you get stuck with the same problem…
https://blog.leitwolf.io/approve-a-sharepoint-document-with-flow-set-content-approval/