Select All Layers

In MS-Win Version of Photoshop (25.6.0 Release), I attempted to select all layers of PSD files then merge them.  But, through

1. Menu Bar : Select > All Layers

2. Hotkey combination : Alt + Ctrl + A

None of these methods worked in selecting all layers (no response at all).  I have to select the layers on the Layers Window by the mouse and activate the short menu to complete the merge layers.

Is it a known bug?  Or, did I miss any steps?  Kindly advise.