When I open the terminal, for some assignments, it has onepned normally and I was able to zip and download them but for others, the terminal doesn’t open right, and when I try to zip and download, it doesn’t work.
# tar -czf ~/workspace.tar.gz ~/work
mv ~/workspace.tar.gz ~/work/workspace.tar.gztar: Removing leading `/' from member names
tar: /root/work: Cannot stat: No such file or directory
tar: Exiting with failure status due to previous errors
#
If you are using the instructions from the Coursera Help Center, please realize that they are too generic and don’t really work as written on these courses. Have you looked around at what is under ~/work in your case? I’ll bet you find nothing. How much linux shell chops do you have? Here’s a command that might give a clue where the assignment files actually are:
find / -name "*.ipynb"
Also note that just downloading all the files is only the first step. You’ll need to install quite a large amount of software on your system if you actually want to run locally. Also note that it’s not my job to be your (unpaid) personal IT support staff when you have trouble with that. You’d better be able to spell “google” and be ready to get your hands dirty. 
It also really depends on what your goals are for downloading. If all you’re trying to do is back up your work (which is a great idea), you can just do “File → Download (as notebook)” from the running notebook. Coursera has been known to delete student work for random reasons, so it is highly recommended that you keep backups of all your notebooks.
If, on the other hand, your goal is actually to be able to run the notebooks locally, then you do need all the ancillary files. But as mentioned in my earlier reply, that’s the easy part. You then need to not only install all the required packages, but figure out how to duplicate exactly the versions of said packages to avoid “versionitis” problems. This is a non-trivial undertaking for which there are no complete instructions at this point. 