Nautilus won't snap to left or right [duplicate]

It would appear that this has something to do with the sidebar. My guess is it's a bug.

If you turn the sidebar off, the snap starts working as it should. Try the following commands from a terminal window.
Edit: The commands I've written out below hide and show the sidebar which can more easily be accomplished (as pointed out in comments) by simply pressing F9.

Turn the sidebar off:

gsettings set org.gnome.nautilus.window-state start-with-sidebar false

Turn the sidebar back on:

gsettings set org.gnome.nautilus.window-state start-with-sidebar true

 
While this is an answer, it's more of an option than a solution ... and probably not the solution you are looking for. It's certainly not the fix I or most other people would want, but it gives you a direction as far as reporting a bug!

This problem has disappeared when I upgraded Ubuntu from version 18.04 to 18.10. So unless you want to stay on 18.04 (because of the LTS for example), the upgrade is a clean solution.

Having this very problem with Ubuntu 21.04 after a fresh install both settings app and nautlius file manager having this issue other apps seems working fine, file manager works when side panel moved to buttom

Another solution is to install a different file manager, for example my personal favourite alternative (and very similar to nautilus) file manager is Caja. You can simply type "caja" into software center and download from there. Caja has no snap problems.

However, there are some other applications that have the snap in problem, such as Discord, that this will not fix.