You are using the right feature, but sometimes fillets are fussy. It sometimes happens that you can add a fillet on one side of a symmetrical body, but not on the other side. Sometimes you can use a Face Fillet when a Constant Radius Fillet on the edge fails. Sometimes you can use a Variable Radius Fillet with all the values set to the same value as a Constant Radius Fillet and it will work. Other times you may have to reduce a Fillet Radius by some tiny amount to get it to work. Or you may have to use a Variable Radius Fillet. In extreme cases, you might have to use split lines to generate edges where you want your fillet to end, then make a Fill Surface that gets you the right shape, and then use Replace Face to give you the fillet shape. You may have to use Delete Face if the Replace doesn't work. Sometimes you have to give up and use some other method to tell people that there is supposed to be a certain size fillet in a certain location.
What I found scary is that a CTRL+Q revealed lots of errors in the model. That is why your fillet fails on my machine.
I recommend that you start over from scratch using what you learned from this first attempt.
Make use of symmetry whenever possible to reduce work.
Constrain all of your sketches.
I the technique of creating this model can be significantly simplified.
I can walk you through step-by-step how I might model the part - but this would take some time. Are you willing to make that effort?