What do you mean by "Format". i.e. What data do you expect to see? Do you just mean [File Type], i.e. "M4A"? Or do you mean [Compression], i.e. "MP4 AAC"?
I don't know what you mean by the "drop-down on the r<igh>t-side". That could mean many things. An image showing what you mean would help.
Are you are trying to add a "Format" column to a View, by right-clicking in the header and selecting "Format" from the drop-down list?
I don't think there has ever been a [Format] field in MC. If you had one before, you must have added it as a Custom field. If that is the case, and you recently upgraded to MC24, did the upgrade find the previous version and load the previous Library, or did you re-import all your music? If MC didn't find the previous version, you should have made a backup in the previous version and restored it into MC24, and any Custom field would have come across.
If "Format" was a custom column in a View created using an expression, then a restore of a backup should also bring that across.
