What causes "attribute key cannot be found when processing" errors?
In my case, I had 2 tables: Sales and Company.

I had one record 'ABC' in Sales, but it was supposed to be in Company. Because of that, when I processed the package I got the same error.

To fix it, I just created a row with '0' Id  in Company, and kept all the columns in that particular row as unknowns. After that, I processed the package and it worked.