It might happen that you identify that a primary key is set incorrectly for a table. What happens fairly often is that users try changing the primary key in the output mapping, however, in this case the transformation fails with a following error:
This tells you that the primary key in storage is specified differently than it is in the output mapping and hence the table can't be loaded from the transformation to the storage. What needs to be done in this case is to remove the primary keys from the table in the storage, then add the correct primary key in storage, and just after that run the transformation.
You can adjust the primary key on a table in storage if you open its detail and then click the little pencil in the "Primary Key" row:
Once the primary key is set to be the same as is set in the output mapping, the transformation will pass.
More details available here.