It looks like some multiprocessing code is attempting to pickle some data to send across processes, but boto3 resource can’t be pickled.
Similar to this comment: PicklingError encountered when using multiple GPUs · Issue #67681 · pytorch/pytorch · GitHub, you might want to try:

Passing arguments and construct boto3 client in the subprocess
Use/cache/store results of using boto3 APIs in some format that can be pickled, and pass around the result.