Hi salamala, can you please post the 3DM model file so I can take a look at your actual geometry instead of only screenshots?

There are many different possible problems, and it's hard to know which in particular you're running into just from a screenshot alone.

Some ideas are: do you possibly have some curves in that same area stacked up on the same location as the edges? If so then you will need to hide those so you can select edges.

Also if those surfaces are coming together at a very slight angle to one another rather than being smooth to each other, that can make for difficulty in filleting in situations like that as well.

It helps a lot if you can post the 3DM model file with your question though.

- Michael