This error generally occurs when we use Unmapped fields in data entity (Data Management) and try to map manually through map source to staging option. The main reason of this error is that Unmapped fields are virtual and computed. Unmapped fields always support read actions as per Mircosoft documentation for unmapped fields. But in some cases we need to do write actions on Unmapped fields we can achieve it by doing following steps.
1- Error (Unmapped field):
2- Error resolution is to set "Allow edit on Create" (Property) to "Yes". This will enable write operation on your unmapped field.
3- By setting the property Data Management will automatically map your Unmapped field.