Hi Lara, fillet won't work on those edges since they are already rounded, although the rounding is squished down to only a small area.

You can't generate a filllet on an edge where the surfaces meeting there are already smooth, you can only put edge fillets on sharp edges.

Generally if you want a slightly rounded end cap it's better to build one larger curved surface using something like sweep or loft and have that cut the end of your object off using it as the cutting object in boolean difference:









- Michael