Some organisations have made the constituent id a required field. Unlike some required fields such as, for example, the constituent last name, where made required, the constituent id behaves slightly differently.
The constituent id is only actually required for previously saved constituents. This means that in reality a new record can be left blank and the it will save with RE generating the constituent id.
Unfortunately Importacular is not aware of that behaviour and treats the constituent id exactly the same way as other required fields. As a workaround if your data source does not have a value, you should put any value in the default area (or map it it to any field). Then on the field settings area mark the record so that it never updates the existing or the new constituent as shown below.
(Note that the screenshot is taken from v3.0 which may look sightly different from earlier versions but the two checkboxes should still be unchecked)
We plan on changing this behaviour in a future version so that if the constituent id is generated automatically Importacular will not mark it as a required field in the mapping. However at the time of writing the latest version is 3.2 patch 2 so the above solution should be used as a workaround.