Command J is not working

the command J is not working neither with short cut or Path menu. it was ok but in the middle of my working process it is not working anymore. could someone please guide me?

What happens when pressing ⌘J? Is there anything on the background or just parts of what you expect to be there?

Make sure that what you want to move to background is selected.

nothing is in background and I select all a character and then press command J to have the whole character in background. it always worked but suddenly stops functioning.

How are you looking at the background?

The red line showing the background shapes can be activated and deactivated with ViewShow Background. Maybe you deactivated it by accidentally pressing ⇧⌘B?

Switching to the background (⌘B) always shows the background shapes.

yes, I mean selection to background.
It is strange but it’s fixed by save as and replace the file.