I am facing same issue. It has become incredibly difficult to test message exchange between two users on iMessage extension. It has been an year and there is no solution still... I hope they come up with solution in iOS 14 😟
iAnni
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I was getting similar error for iMessage extension app i.e. Large App Icon assets must have the dimensions 1024x768. Although I previously published the app to store successfully and there was no change. Dimension was alright and there was no alpha. As trial and error, I ended up doing ENABLEONLYACTIVERESOURCES = NO and ENABLEBITCODE = NO, and it worked.