The input for the "genome" field is not another BAM file, but a file of coordinates such as those in interval, bed, gtf, etc. format. Appropriate formats will be detected and appear in the pull-down menu (as long as the "datatype" metadata is assigned).
The public Main Galaxy instances are down for scheduled maintenance today, so you can try this again later tonight or tomorrow. Please follow here for updates: https://status.galaxyproject.org/
Thanks, Jen, Galaxy team