Current LEGO Digital Designer download is missing a lot of parts
Unfortunately LDD has been actively deprecated for quite a number of years and LEGO officially noted they would not be updating the parts set and would, in fact, be removing some parts quite a few years ago.

The only solution is to use one of the alternatives such as any of the options that use the LDraw libraries. One of the more popular right now (for good reason IHO) is Stud.io, which as of the completion of the BrickLink acquisition will be owned by TLG.