Cannot Bevel top section of device

Hi Rimi ,

Select the whole Mesh and then: M > Merge > by Distance.

Followed by SHIFT+N.

That will probably fix it.

If not, it would be easiest if you could post a link to your .blend file (via Dropbox for instance).

Each Face has 2 sides, a front and a back. when a model has Faces that are not 'aligned', I mean a back Face next to a front Face, weird things might happen (not only when Beveling):

In CG we often speak of Normals when discussing the direction of Faces. SHIFT+N makes those Normals consistent.

Shift + N is the hotkey for the "Recalculate Normals" operation. You can also find it under Mesh -> Normals where you'll find some other helpful options.