Length in Tag Panel

Please make provision to add Lenght under "Tag Panel"

may be
%_length%
%_length_seconds%

However under Fields it it showing "LENGTH", but nothing happens when choosen

Please look in the feature and guide me too

The field "LENGTH" without underscore (_LENGTH) is a user-defined field which has to be filled by the user.
It may be worthwhile to reset the list with the functions in the right-arrow-button next to the dropdown list
In general, the tag panel allows only fields that can be edited by the user. _LENGTH is set during the creation of the file and cannot be modified by MP3tag.