The ruck had formed. Had it?

Quinton Geldenhuys of Italy drives ahead. John Barclay tackles Geldenhuys who falls beyond Barclay. Mauro Bergamasco and Leonardo Ghiraldini arrive and crouch over Geldenhuys. Barclay gets to his feet and gets possession of the bball. The referee penalised Barclay for being offside. He says the ruck had formed and makes a tent of his hands to illustrate the fact, in case the Scots do not understand.

But had a ruck formed?

Definitions are important. It is important to use them to identify the phase of play before him.

Law 16 DEFINITIONS
A ruck is a phase of play where one or more players from each team, who are on their feet, in physical contact, close around the ball on the ground. Open play has ended.

Is this what happened? Are those crouching Italian players in physical contact with any Scot?  Not really, unless you count Chris Cusiter's unaggressive hand on Bergamasco's back.

Apart from that there was simply none of each team on their feet in physical contact.

There was no ruck.

Wrong as it may look, Barclay was entitled to play the ball.