Can't type & modify anything in my script with monodevelop

Hi,

So after I create a script, I am able to open the script using monodevelop but I can't make any change with it from there. Seems like most of other functionalities are fine, but nothing happens when I type in the script. I can move around the cursor but I can't type or make any change with the script, except selecting a block of text and deleting it (and plus the revert using cmd + z), which is weird for me. When I modify the script using other editors, it notifies me that there has been a change with the file and asks me that whether I want to reload the new file, and if I say yes, the script is changed to the newest one, but I still can't make any changes from the monodevelop editor.

I have tried to look for the answer and I have seen many questions regarding the failure of opening scripts, debugging or other things, but I have never seen any post that has the same issue as mine.

I am using Mac and my version is El Capitan. Thank you in advance.