Looking at the issue reported above, there are couple of possibilities that you haven't been able to combine files.
1- To combine files, you would require subscription for PDF Pack or Acrobat DC. Do you have any of them?
2- The documents you are trying to combine, are either restricted or corrupted. Secured documents doesn't let you to combine or make any further changes. Open document>right click Document Properties>Security >Check document restriction summary.
3- If you are working on Document Cloud web then, try logging in using different web browser.