Deleted items don't go into trash On MacOS, El Capitan, any file I delete from Finder does not go to the Trash, it is deleted completely.  When I delete a file the warning message says "you are about to permanently delete the file".

So it seems as though, the Trash has been disabled - how do I renable it?

The Mac is a Mac Mini, Core i5, 8GB RAM and 500GB SSD.
There are two likely possibilities:

First, you're accidentally holding down the Option key or it is stuck - do you get the same behavior if you use a different keyboard?

Second, the permissions on your ~/.Trash may have become corrupt; run Disk Utility from your /Applications/Utilities folder and verify permissions on the drive containing your home folder - if errors are reported, restart from Recovery Mode and run Disk Utility again.