Something's amiss here. I used the calculator for a flyback converter that you suggested yesterday: http://schmidt-walter-schaltnetzteile.de/smps_e/spw_smps_e.html
With 24V input, 300V and 200mA output, the calculator suggests 120uH in the primary and a turn ratio of 80...
Hey, that website is one I often recommend to SMPS beginners but it is giving me a "bad request" error on most (but not all) of the sections. Is it just me, or is it broken for others?
Yeah, I'm not going to build the flyback smps anytime soon. I'll make sure I do proper research before I do that.
The asc is attached, thanks
Right, a bunch of things wrong just at first glance. The transformer phasing is wrong - for a flyback the dot ends are inverted w/r/t each other - and you don't include the H when specifying the inductance, so the primary should just be 120u, not 120uH (you did specify the secondary correctly).
Next big issue is there is no RCD or Zener clamp across the primary; switch destruction is guaranteed in the real world, but LTSpice apparently doesn't care...

You also need to specify a diode on the secondary with an appropriate voltage rating; a 40V or whatever Schottky ain't gonna cut it with a 300V output.
The pot isn't a standard LTSpice component and simulating 3 seconds is about 2.99 seconds too much (quite literally: 10ms should be enough here). I like to set the simulation time to capture 100 to 1000 switching cycles, or 4-10 AC mains cycles, whichever applies.
Probably some other things but this should be enough to make your brain hurt for awhile.