Is this for BFV? BFV does not have the flare shader (nor can it be added back since it doesn't load a shaders rfa anymore). It pertains to the TSun LensFlare present on all vanilla BF42 maps. The LensFlares have no effect ingame anyway. To fix that error, remove all the coding that refers to the "TSun" object. It should then solve your missing flare shader error. None of the BFV maps use the LensFlare object in the skyandSun con files like BF42 did. Thus if you add one back like from a converted map from BF42, you will get that error. Look at an existing BFV map to know what you won't need in the con file after you port it.
No it is not for bfV it is a conversion from bf1942 vanilla to bf1942 SW demo.. I created a custom map and am now tryiing to convert it into the demo. I tried your advice though and instead of getting an error the game simple closes.
Since I don't work with the demo I can't help further on this. What you can try is making sure you didn't corrupt the shaders.rfa. Even BF42 demos should have the flare shader.