TL;DR: If you have an app version that was approved for the official app store, you cannot upload a build with the same version to TestFlight. You have to remove the app from official app store submission, or increase your TestFlight version #.
I figured out why this happened to me.
I have an app in the app store with version 2010.
I have been running an external beta test with version 2020, and build numbers in the range of 400-410
In order to discover if my app would get rejected in the app store once I'm ready to actually release it in a few weeks, I submitted version 2020, build 407 for official app store review. I expected it to be rejected for various reasons. It was approved! However, i'm not ready to release it since I still have more fixes I want to implement. So, I let it sit there, waiting for me to do something with it.
Then, I wanted to upload a newer build for my External beta, still version 2020, build 410, and I received this error.
Thanks to user462990's answer, I realized that the error could be due to the fact that I have version 2020 currently approved and waiting to be launched into the official app store, so iTC didn't allow me to submit the same build # for a TestFlight external beta.
So, I selected "cancel this release" of the official approved version, and Viola! I can now upload a build with the same build #.
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