The job likely ran out of memory during execution. BAM datasets over 25-45 GB (content matters) approach the maximum size for data conversions/upload.
There is usually no need to do this conversion within Galaxy. Tools within Galaxy do not use the
file.bam.bai as an input specified independently (a selected BAM dataset is internally linked to the BAM index and entered as a single BAM input).
Should you instead want the index file to work with outside of Galaxy, directly download it from the primary BAM dataset. Click on the disc icon for the BAM dataset, and you will find multiple choices, where both the BAM and BAM index is included.
Thanks, Jen, Galaxy team