It is not clear what tool is presenting with an error. DE analysis is covered in much detail in the Galaxy Tutorials and might be able to help you to solve the problem.
Troubleshooting overview:
Troubleshooting FAQ, including how to check for input problems:
The subsection of that first post that covers using tutorials to troubleshoot usage problems. You’ll find DE analysis under the “Transcriptomics” tutorial group.
Hope that helps!