JIRA automatically creates entries for it in all existing field configurations. Also, when a new field configuration is created it is created with entries for all fields existing in the INSTANCE, EITHER system or custom. 

This means that it is never necessary to create an entry for a field in a field configuration, as they exist from the moment either a custom field or a field configuration are created. 

Project Configurator takes this fact into account, and if it detects a situation where it would have to create an entry for a field in a field configuration considers it an anomaly and stops the IMPORT with that message.

The solution is:

1. Maybe the referred custom field is a locked field which must be created by a plugin/application such as JIRA Agile/Software. In this case, ensure the right version of the plugin/application is installed at the target instance.
2. Verify also that it is installed with the same locale it had been installed at the source instance. The ultimate goal is that the set of JIRA Agile/Software custom fields is exactly the same in both instances.
3. If the field had to be created when the configuration is imported, verify that the import of custom fields was not disabled
4. If it is a time tracking field, CHECK that time tracking is ENABLED in the target instance. Otherwise, the time tracking fields are not available.