How do remove administrative rights if the checkbox is disabled in Users&Groups
I just did it by using the command line way. I am not sure if this is recommended or not since I have no ideas why I can not do it in the GUI. 

sudo dseditgroup -o edit -d admin -t user admin


The above command does the job.

PS: Because it's a second hand Macbook pro and it's the first MacOS I ever used. I thought admin was a built-in user which I could not simply use the above command. But it turns out I was wrong and admin is not a built-in account and I can simply run the command.

Btw, for your info, if you want to do this for other usernames:

sudo dseditgroup -o edit -d &lt;username&gt; -t user admin