Sorted it, under sharing and permissions my account was not there only sysyte, wheel and everyone. Added my account set to read and write and now works.
They were always ok, just not letting me write to root folder, could write to home folder ok also. Yours sounds like you need to repair permissions
For a user to NOT be able to write directly to the root of the boot volume is normal. At least in Mac OS, BSD and Linux and most UN*X (if not all) systems.
In fact, Mac OS Sierra was the same way. And if you were able to write directly to the root of your boot volume as a standard (even Administrative) user without authentication would mean that you or someone or something else changed that permission at an earlier date (maybe it inherited lenient permissions from a prior Mac OS version, I can't tell you).
I have a 1 month old MBP that shipped with Sierra and is still running Sierra and it acts just like this, I must authenticate to move a file to the root of the filesystem.