Is it okay if I write "il est pas important"?what is the difference between the use of "ce" and "il" here?
When saying something "is NOT" you have to include both 'ne' as well as 'pas' surrounding the verb that is not being done. In this case, the 'ne' preceeds 'est' and is contracted to become "n'est"
"Ce n'est pas important" can either be "It's not important" or "This isn't important" depending on the context (which is the subject).
Since there isn't any context, "Ce n'est pas important" can mean both.