Fatal error: Call to undefined function ctools_include()  While implementing Drupal 6 Multistep Form Using Chaos Tools
As the Chaos tools suite module defines ctools_include() in ctools.module, the only reason I can think of what would explain the error is that you didn't install the module. The other reason could be that you are trying to access the function when the module is still not loaded; for example, if you call the function inside hook_boot(), then you would get that error.