I am unable to reproduce this at Galaxy Main https://usegalaxy.org. The server is running Galaxy version 18.05 and I tested with the most current version of this Collection Operations tool.
This type of error can sometimes indicate a transient server-side issue that a rerun might avoid. So, try that first.
If it still reports a problem with a rerun, and you can reproduce this at Galaxy Main, a shared history link, along with the datasets numbers involved can be sent in. This is how:
Support FAQs: https://galaxyproject.org/support/
- Reporting Usage Issues or Software bugs https://galaxyproject.org/issues/
Thanks! Jen, Galaxy team