18 months ago by
The tool is probably expecting a two column file listing the chromosomes and lengths and it is not found in the history as an active dataset with the dataype assigned as "tabular".
What exact tool is this coming from in your case? The complete name including version (at the top of the tool form) would be helpful. If you do not find a data problem, there could be a tool problem and we can check that.
Are you working at http://usegalaxy.org or somewhere else (where)?
If a local or cloud galaxy, what version is running/source?
Thanks, Jen, Galaxy team