First create the pattern, then the Blend.
When using the Blend feature for patterns please regard to use it on the origin geometry.
Blend on origin geometry is supported.
Correct use: Command "New Chamfer/Fillet..." is displayed
Blend on derived pattern geometries is not supported.
Incorrect use: Note "Face is a result of pattern feature" is displayed
When following the correct procedure, the Blend feature is applied to the origin geometry and all derived pattern elements.
Result
For patterns, if problems occur, try both edge selection and face selection.