You have snap grid enabled also. Only use the required snap tools for any one job.......

Don't get frustrated here, this sounds like a Printer/slicer problem, use another forum.

Basic Attachment Tutorial

More specifically:-

https://wiki.freecad.org/Basic_Attachme ... Attachment

Attach it to the sketch for that pocket, mode concentric.....

If the pillars have to be separate bodies, then use a draft array instead of a PD pattern.

Single solid rule, you have to add the pillars to the pattern so it will succeed.

Perhaps I wasn't clear, it is only available in the dev version (FC 0.22)

It's not available in your version upgrade.

what is a folder, I don't have any folders?

It is a tutorial, it describes principles. It requires some extension on the part of the reader, not unusual for such.

There is a typo above, here is the page Basic Attachment Tutorial

After it worked through, I can answer any queries.