Hello, I'm not sure if i have understood (I'm French) this rule :

• "Entries must use open licenses. Licenses must allow for free use, including royalty-free commercial use. "

Can we or can't we use licenses like CC BY-NC-SA ?

CC BY-NC-SA is too restricted. It does not allow for free use. CC-0, CC-BY, CC-BY-SA all qualify.

It all depends on what you call "free use". Forbiden commercial use isn't agains free use, it's against "commercial" use. But Okay, it's your jam. I just think the title "free open source" is misleading. "Free open source for commercial use" would be more exact.