Consider the following before doing the bevel:
1) Remove excess vertices – In Edit mode, select all vertices and then press Alt-M (if Blender 2.83 – just M). Choose “by distance.” Do the bevel again.
2) Normals facing same direction – In Edit Mode, click on the Viewport Overlays pull down menu. Under geometry, click on the Face Orientation checkbox. If across the bevel there is a combination of blue and red normals, select the red normal faces and press Alt-N and select Flip. All should be blue. Uncheck the Face Orientation checkbox. Retry the bevel.
3) Apply Location / Rotation / Scale – In Object mode, click on the object and press Ctrl-A. Apply the Location, Rotation and Scale. Tab back to Edit mode to bevel.
Hope that one of these helps.
