Unable to create temporary items in the proposed location

Upgraded to Sierra, lost OmniPresence.

When going through the process of creating a new synced folder I get the dialog:
“Local documents folder cannot be used. Unable to create temporary items in the proposed location.”

I am trying to install on the top level of the same external drive that has been home to OmniPresence for a couple of years. No changes of which I am aware.

Curiously, it appears that I am allowed to install on the iCloud Drive or the desktop. iCloud Drive is taboo and the desktop does not fit my purpose.

Thoughts?

I reply to my own post: The next morning all worked as it should have. I was allowed to create an OmniPresence folder where I wanted it. All is well with OmniPresence and Sierra.