Hello @Noer_Barrihadianto,
I can see that you have done very careful checking along those steps, but if it is really that there is some metadata stuff being corrupted when it being saved on Colabs or your local environment, the quickest way to correct it is to start over with a fresh assignment notebook, and then copy your exercises’ solutions to the refreshed notebook and submit.
If you would like to try it, please follow these instructions which will ask you to rename your current notebook in one of the steps, before the system can get you a new notebook. Then you can copy your work to the new notebook.
Raymond