The block editor is for editing of dynamic blocks. Right click on the block and select "Edit Block In-place" to edit plain blocks. One limitation with the block editor is that it is only for 2D.
The BLOCKEDITLOCK system variable can be set to 1 and that will result in the following behavior and the Block Editor cannot be opened:
If the Bedit command is run when BLOCKEDITLOCK is set to 1 this message will show:
** BEDIT command not allowed. Block Editor is disabled. **
If you want to be able to double click on a plan non-dynamic block and edit it in-place use CUI and change the Double Click Actions for Block. Use ^C^CRefedit as macro. The original macro is
in case you want to restore it: $M=$(if,$(and,$(>,$(getvar,blockeditlock),0)),^C^C_properties,^C^C_bedit)
This will be the result: