Hi Clare! Welcome to the community!
I can think of a few different ways you could create a printable certificate in Storyline 360 itself.
One of the easiest options would be to customize the visual design of the course's Results slide and make it look like a certificate. Because you can add a print button to that slide, that makes creating a printable certificate take no time at all.
If you need a different solution, though, here are two other options that have previously been shared here on E-Learning Heroes: