A similar problem was reported in ticket #340:
https://cp3.irmp.ucl.ac.be/projects/delphes/ticket/340
The root2lhco converter needs the Track and Tower branches.
If the root file that you need to convert to LHCO does not contain these branches, the easiest solution would be to regenerate this file with all the required branches enabled.
In more recent Delphes releases, lhco2root converter checks if the input ROOT file contains all required branches. Otherwise, it cleanly exits with the following error message:
** WARNING: cannot access branch 'Track', return NULL pointer
** ERROR: ROOT file doesn't contain all required branches