The feedback loop of an oscillator must be opened for measuring or simulating the loop gain. This is necessary to show resp. prove if the circuit meets the oscillation criterion (Barkhausen: Loop gain equal resp. larger than unity).
For this purpose, the loop must be opened at a point where this opening does not change the loading conditions during closed-loop conditions.For the present case, it is the best choice to open the loop at the opamp output.
The loop gain will be, of course, NOT be zero. However, you must connect, of course, an AC signal voltage at the opening (and NOT a DC source as shown in the figure) and measure the opamp output. The frequency of this AC source must be varied within a range that, of course, contains the expected oscillation frequency.
(The Photon has "beaten" me by 3 minutes)