The default licence applied to a post is CC BY-SA 4.0 which you are free to apply to your own works. However, you won't be able to post other people's works unless they are already available under a compatible licence.

In cases where you know the original source you can select a compatible licence. In this case, since you don't have knowledge of the original source and how it was licensed, you won't be able to post it here.

I'm not a lawyer, but I believe copyright only protects presentation, not puzzle mechanics. So making your own similar version of a puzzle may be fine, but uploading an image of an existing puzzle will in general not be permitted unless the puzzle was originally licensed in a way that allows for this.

I'm making this as a general point, since you mentioned not knowing the source. In this particular case, the presentation of the puzzle is so minimal and so similar to other standard diagrams that it's unclear whether it could be considered to have copyright. I don't know where to draw the line, but if you wanted to be extra safe you could upload your own presentation instead.

trichoplax‭ The screenshot of the puzzle that I downloaded a few years ago was too big to upload here. So I created a replica image using an app on my iPad. I uploaded my replica image not the original screenshot.

trichoplax‭ I posted a question on meta.codidact about the issue you raised.

The question can be found here:

https://meta.codidact.com/posts/295960

Hopefully a staff member will respond. If necessary I will revise or delete my puzzle post.

If you already recreated the puzzle in your own style then you're a step ahead of me. Good idea raising a Meta question. I've added the support tag since you're seeking an official response.

trichoplax‭ I finally found the source of the puzzle and have added attribution.