Assume Role is not available for Spark Jobs until Talend Version 7.0.1.
Upgrade to version 7.0.1 to resolve this issue.
As a potential workaround, perform the following steps:
Create a Big Data Batch Job.
Create a Standard Job.
Add a tRunJob component into the Big Data Batch Job and select the Standard Job.
Add a tS3Connection component to the Standard Job where Assume Role is an option.