Beveling with a bad topology of an imported model ? in my model i have awful triangulation that is making my bevel not working at all. ( i just want a bevel to this front face right here , the downside L) 

normally what i do is just dissolving the unwanted edge and voila i have a polygon an my bevel work again

this time it doesnt work and i cant get rid of this , i dont understand why ,
decimating with the planar option at 5 degree also dont work. 

here is the file 
https://we.tl/t-Vow8tge8En
You don't only have edges crossing your external faces, you also have several inner faces that should not be here, so maybe if you delete them it will fix your problem.