Hi Arian, welcome to the forum!

Can you also please attach the 3DM model file so I can examine your object in more detail?

Also what fillet radius are you trying to use? The first thing to try is to use a smaller value, maybe going down by 1/10 in size each time.

One of the easiest things that can cause filleting problems is if you're asking for a fillet radius that is too large for the fillet to actually fit in the available space...

But there are also a whole lot of other kinds of things that can also cause problems as well, like geometry that bunches up into tight folds or has a mix of smooth and sharp areas along shared edges (smooth edges makes fillets disappear), and various other things.

It's hard just looking at a screenshot to know which of these various things you might be running into.

Also the geometry library that MoI uses can have difficulties handling certain kinds of corner patch situations where different edges come together requiring a complex juncture to be formed in between them. Some other CAD programs can handle a lot more corner situations more easily, so some people use other programs in combination with MoI to help with filleting, ViaCAD is good companion for that.

But if you can please post both your 3DM model file and also let me know what fillet radius you are trying to use, that would help to give you some more specific advice.

- Michael