Looks like it’s the same issue as here: Learning Agents fails - #2 by Deathcalibur
It looks like you’re running into a timeout when doing the experience gathering loop. You can verify this by printing out the response using the steps in that reply.
You should be able to resolve this by going to Program Files\Epic Games\UE_5.3\Engine\Plugins\Experimental\LearningAgents\Content\Python\train_common.py and increasing the timeout on line 228 in shared_memory_recv_experience_multiprocess
We will work on making the error messages clearer for this type of issue and seeing what we can do to make this easier to avoid.