Length in Tag Panel

Please make provision to add Lenght under "Tag Panel"

may be

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.