Maybe if you add a comma. I can imagine a prior sentence. “Are we going to reach our goal? If not, make it happen!”

Depending on the context, you probably need a comma. Has the meeting happened yet? If not, make it happen!

Hi Emily! The wonderful folks on italki have helped with correct punctuation by pointing out that a comma is needed here. Considering punctuation: "It's time to cook, grandma." versus "It's time to cook grandma." As you can see, punctuation can make a big difference in the meaning. Have a great day folks and stay safe.

You should say: if not, make it happen

As others have said, the comma is very important. Otherwise, it doesn't make sense.