Since the dawn of CC 2017 for After Effects, the ability render out videos in a .mp4 format have been taken out. By taken out I mean taken out of default render options, there is still a way to get that sweet nicely encoded mp4 format.
Now if you have the creative cloud you can download Adobe Media Encoder, this is what is gonna let you render out in .mp4. The best way to go about rendering something out in this program is to, save your After Effects file, click on the comp that you want to render, go to file – export – add to Adobe Media Encoder Que. Media Encoder will eventually open up, and if it doesn’t, go to wherever you installed it on your computer, go down to the .exe file and open it, once opened drag and drop your comp into the application. There will be blue text that appears, that’s where you can change your options where to render, and the type of file extension you want it to have.
The Render 