I have never seen that particular error message. If you are using the course with your browser language set to something other than English, it might be worth opening a different browser with language set to English and try the submit again.
If that doesn’t change the result, then the next thing to try would be to get a fresh copy of the assignment and try again. Just rename your existing notebook first, so that you can copy your work over to the fresh copy. Here’s a thread which explains how to do that. That is linked from the DLS FAQ Thread, which is worth a look on general principles if you haven’t seen it yet.
Once you have the fresh copy, just carefully “copy/paste” over only your changes from the “YOUR CODE HERE” sections and then run it to check everything. Then “Save” and “Submit” that from the “English” browser session and let us know if it still fails.