Without detailed information about your project, in particular what specific output settings it may have used, nobody can tell you much. And the usual essential courtesies like providing system information wouldn't hurt, either.
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If you're trying to output to formats such as MP4 with H264, they are no longer available in the newest versions of AE. Instead, try to use Adobe Media Encoder. Here's a preset for the Encoder if you'd like that:
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