important preposition

This looks a bit foreign to me, that is, written by a non-native speaker.
I'm not sure what a make-up exam is.

Maybe this is what is meant:

No supplementary exam will be offered. For that reason, the next lecture is important.

(just a guess)

It's American. A make-up exam is an exam that some instructors allow students to take if they missed or failed a previous one.

"For" is indicating the object, "it," of the activity, "the lecuture."

Thank you, McTag and Infrablue.