Nonsense. Remains of both engines and lots of debris were recovered at the Pentagon, along with the remains of everyone aboard (except one infant) as well as their personal effects and luggage and of course, the Flight Data Recorder for Flight 77. Additionally radar tracks from 6 different sites confirm the FDR data and positively tracked 77 from take-off to impact. Plus you have to figure the plane took off and never landed anywhere with no one aboard ever seen again, the airline admitted it was their plane as did their insurers - in spite of the negative financial implications of doing so. Prima facie it was Flight 77.
Conspiracy Theorists complaints about the Pentagon attack typically revolve around a couple of myths and fallacies. One is that they for some bizarre reason expect there to be large, easily recognizable intact sections of airplane visible on the lawn - wings, tail, fuselage sections, etc,... This I suspect is because most airliner crashes occur at low speed (120-150mph) during landing or take off with the aircraft skidding across the ground:
https://www.toledoblade.com/image/2013/07/06/800x_b1_cCM_z/APTOPIX-San-Francisco-Airliner-Crash-7-6.jpg
http://airlineworld.files.wordpress.com/2008/06/taca_accident.jpg
http://www.baaa-acro.com/photos/MD81-SAS-Stockholm.jpg
Flight 77 however flew at over 500mph head-on into a 2-foot thick reinforced concrete wall with an additional layer of Limestone cladding. No large pieces are going to survive that. It will look a lot more like this:
https://share.sandia.gov/news/resources/video-gallery/index.html#rocketsled
The other common complaint is that either the entry and/or exit hole in the Pentagon is too small or the wrong shape or whatever. Personally I blame Bugs Bunny. If you were like me you grew up on a steady diet of Saturday mornings filled with animated animals crashing through walls leaving say, perfectly duck-shaped outlines in them.
http://911fantasyworld.files.wordpress.com/2011/04/daffy-duck-hitting-wall.jpg?w=300&h=336
I think that is what some people expect to see at the Pentagon
http://911fantasyworld.files.wordpress.com/2011/04/plane-hitting-brickwall1.jpg?w=300&h=155
But real life isn't a Warner Bros. cartoon and a crashing jet doesn't punch a cartoon-like outline of itself into a reinforced concrete building,
While the wingspan of a Boeing 757 is 124ft the entry hole in the Pentagon was estimated by ASCE as 75ft wide - precisely the span of the aircraft from engine-to-engine. Why? Because the outer wings were too light and thin to penetrate more than 2 feet of reinforced concrete and stone.
CT's also like to complain about the lack of clear video from the high-definition, high-speed, skyward aimed video camera's they ASSUME were literally spread every few feet at the Pentagon. This is nonsense of course. The Pentagon is not a shopping mall. Camera's may deter shoplifters but real security is done by boots on the ground and in 2001 that is how the Pentagon did it. Besides, that line is just a ruse to distract people from the overwhelming evidence I briefly covered in the first paragraph.
I sincerely hope this helped.