How to merge multiple compositions to finished video

I did this how the others suggested but has anyone had an issue with one of the compositions appearing blank when you're trying to preview it? I have to make a demo reel of animations I've done for class but one of them is blank when I run it. I don't know if it's because I used duik to make a walk cycle of what?

Rob and MJ's suggestions work but they come with some problems. If your compositions take a long time to render and they are complicated there is always the risk of a render failure. Nesting all of the compositions in a project in a new master comp and rendering them all together assumes that you have met all of your production standards for sound, color grade, transitions, and timing. Even the best editors cut shots together many times and fiddle with the sound. 

After Effects is not an NLE. Premiere Pro is. After Effects does not directly compress files for delivery to streaming services (YouTube, Vimeo, Instagram, Facebook, or even for sharing by e-mail) using the Render Queue/Output module. You have to use the Media Encoder. The Media encoder runs a copy of After Effects in the background when it is rendering compositions so the AME is almost always significantly slower at rendering a composition than the Render Queue is. The standard for delivery to the public right now is H.264 in an MP4 container. H.264 is. a MPEG format that should be rendered to industry-standard frame sizes and frame rates. The frame size must be an even number of pixels high and wide. Color is always compressed in blocks of a minimum of 4 pixels. The movie is also compressed using Intraframe compression which means that you have an I frame, which is a highly compressed and lossy frame using the original pixels in the frame. The next frame is a P frame which is a highly compressed and lossy frame created by predicting where the next frame's pixels are going to move by looking forward, then a B frame which is highly compressed created by predicting where the pixels of that frame has moved by looking forward and backward one frame, then another I frame. It's amazing that it works as good as it does, it requires resources to decode and turn into a movie, and all of the compression and prediction is one reason that folks end up with video files that have glitching problems when they try and use them as source files for production. H.264, and every other delivery format is also limited to 8-bit color and there are only a very small number of these MPEG compressed formats that make any attempt to include an alpha channel. They are designed to have small file sizes and save bandwidth. 

The primary use of the Render Queue is to render visually lossless production masters. The codecs used are called Mezzanine codecs or Messanaine formats by professionals and the rendered file is a DI or Digital Intermediate, or Production Master. These files are Interframe compressed, can, except for a few specific codecs, be any frame size, any frame rate, and they are much easier and faster to decode when read as a source file in all production software. Almost all, and certainly the DI formats used by industry professionals are capable of 10-bit color and many of them support Alpha channels and YUV color.  

The efficient way to handle a large, complicated After Effects, Or Davinci Resolve, or Nuke, or C4d, or any other 3D or visual effects app is to render a DI and send it to an editor to be cut into a final movie using an NLE like Premiere Pro, Avid, Final Cut, or even iMovie or Windows Movie Maker. 

Most of my projects are movies (videos) that contain a lot of shots. I can't think of more than three or four times in my very long professional career that I have produced a movie that was longer than a few seconds and only contained one shot. I recently cut a five-minute promotional piece for a client. The edited video in Premiere Pro, not including graphics, had seventy cuts. The visual effects and animation part of the project was 2 AEP (After Effects) projects, one for animations, one for visual effects. The animations AEP had 9 comps, the Visual Effects project had 22 main comps. None of the VFX comps were longer than 5 seconds and none of the Animation comps were longer than 8. Most were less. Every one of the VFX comps was started using Dynamic Link in Premiere Pro (which was undone right after the Comp was created by Premiere Pro so I did not have to work on frames that would never be in the movie, and rendered as a DI using my favorite movie format GoPro Cineform 10 bit or GoPro Cineform with Alpha - 10 bit because it renders very quickly, and then the original shots in the Premiere Pro timeline were replaced using the rendered DI. The delivery file was rendered as an H.264 MP4 for the client from Premiere Pro and a Production master, using my favorite DI format was rendered for the archives so it could be used in future productions if needed. 

That's how you work efficiently in After Effects. In most cases, especially if you use a Background Rendering solution like my favorite because it works so well with my hardware, very little time was spent waiting for renders, I could keep working with AE while the DI's were rendering, and the half dozen changes requested by the client on the effects and animation shots could be rendered and replaced in minutes instead of hours or days. 

If you want to become a pro and not just an enthusiast, you'll establish an efficient workflow that starts with a solid and repeatable file naming and organizing system and ends with the ability to make a 20 frame change to a single effect or animation that can be rendered and replaced in your Production master in a few minutes instead of the days that some people spend rendering an hour-long AE comp. 

I probably could have done a pretty good workflow tutorial in the time it took to write this reply. Sounds like a good idea. 

You should have te timeline selected and go to the top of the interface/composition/composition settings and make the length fo the composition long enough to hold all of your comps end to end.

Then drag them from the project panel so that they meet end to end.  (You can hold down shift so that they snap to each other.

Then export the combination of compositions.