This can be a hard exception to diagnose. In essence, Importacular is reporting that the value it was expecting to find wasn't where it was expecting to find it. Often, this is a table entry or an Attribute that was present when the Importacular mapping template was created, but has since been deleted or changed in some way.
If you feel as if your tables and attributes have remained the same for a while, please contact support@zeidman.info to raise a ticket and include in your communication a copy of the template, a sample of your import data and any relevant screenshots you can provide.
Here are two additional guides to help you obtain a copy of your template and a sample of your data.