There are two possible scenarios:
1. You are creating a brand new custom table and you get the error.
A table exists with the same name as the one specified in the Table Name field. In the Table Name field, modify the value to a different name that does not already exist.
2. You need to append data to an existing table or replace the data in an existing table and you get the error.
The Replace or Append box has not been populated.
Enter append if you want to append data to the table or enter replace if you want to replace the data in the table.
This error occurs when trying to upload to an already existing custom table and append or replace has not been specified in the Custom Hierarchy Upload Sheet.