When do you get the error message? When you sync or when you start the program? Is it a Windows error message or a Joplin error message? Is it the same problem where sync gives an error message that it cannot be "activated" or are these two different issues?
Have you ever directly deleted items from your Joplin data store on OneDrive or directly from the C:\Users\<username>\.config\joplin-desktop\resources folder or manually manipulated the Joplin database?
If you are getting the errors from Joplin some debugging data may be of use.
Also is this a previous post of yours and if so what did you do to correct the problem between v2.1.8 and v.3.0.15? Did you correct the problem?
Just trying to get an idea of what is happening and when for those who may be able to help.