Hi,
Cross sections are linked to the branches using their 3 "identifiers": branch name, topo-ID, and chainage. So if the cross sections are not visible on the map (in the Network file), or if you get this error message when launching the simulation, one of these identifiers may be wrong.
For instance the branch name and/or Topo-ID specified in the cross section file may not strictly match those in the Network file. Or the chainage specified for the cross sections may not be in the range of chainages defining the branch in the Network file.
Best regards,
Mathieu