Is the current zero on an opened branch using Thevenin's theorem?
The goal of what you're trying to do is find the equivalent resistance in respect to the points where you removed the resistor.  You are correct in that those 2 are in series, however by doing that conversion and turning it into a 8Ohm you lose one of the nodes connected to where the resistor used to be, and so it defeats the purpose of the whole calculation in the first place.

I would recommend re-drawing the circuit such that A and B are not in the middle of the diagram. That should make things easier to comprehend.

BONUS TIP: 


  The exercise asks the value of resistor R knowing that there passes a current of 1 A, using the Thevenin's Theorem.


You may want to consider why the authors of this problem is giving you this information. Perhaps they are trying to reduce the amount of work you need to do?