Hii, I am trying to simulate a flyback transformer in Lt spice having an input voltage of Vin = 12V , turns ratio = 8.45 , coupling coefficient = 0.8 . As per the calculation, output voltage should come vout = 81.1 V. But its is coming 70V. primary inductances are taken as L1 and L2 = 0.15mH and 10.8 mH respectively. what could be the possible issues in the circuit.

 

I don't think 1V on the base of the transistor would do it much good if it were a real transistor!
I never find SPICE's transformer coupling coefficients easy to understand, I think that even a poor flyback transformer would be better than 0.8
I would suggest using K=1 measuring the leakage inductance and and adding it in series with the primary. I find that easier to understand.

 

The first problem is that your scheme is disgusting. You are controlling a transistor with a voltage source. There is an apologist on this forum for voltage control of a bipolar transistor. You must be a supporter of it. Look at the base current of the transistor. Look at the voltage at the collector of the transistor.