trying to export on media encoder

When you want to use the Media Encoder to render After Effects compositions, the workflow is always faster if you open the Media Encoder before you use the Composition/Add To Media Enoocer Queue function in After Effects. 

Adding to the AME Queue also requires that the versions and builds match. You can check to see if they do in the Creative Cloud app. 

First thing to confirm - are you using the latest version of both After Effects and Media Encoder?
Mismatched versions (say, if you have been updating AE but not AME for a long time) will not talk to one another properly. 

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