Below are the most common reasons why people have trouble importing the CSV file that contains the product shipping parameters data.
The file is not in the correct format
Make sure you have the file saved as a CSV (comma separated value) file. This is done by using the Save As... entry in the appropriate menu of the spreadsheet application you are using. If you are using a Mac, you need to pick the Windows or MS-DOS specialty formats.
Failure to map fields in Step 2 of the import process
Step 2 of the import process requires that you identify the column in your CSV file that contains the data that is identified in on the left hand side of that page. If you left the column headings unchanged after downloading the CSV template, they should be the same.
Product IDs and/or Variant IDs have changed
Columns A and B of the CSV template contain unique IDs that Shopify uses to identify products and product variants. If those IDs have changed since downloading the CSV template, the import utility won't be able to identify what products the shipping parameter data belongs to. Normally these IDs are very static. However, some apps will change these IDs to achieve their objectives. Apps that change the IDs are most commonly related to pricing, such as pricing tiers, wholesale pricing, price breaks, etc.
To determine if your IDs have changed, download another CSV template and compare the Product & Variant IDs in the newly downloaded file to those in the file that you are trying to upload. If they differ, you'll need to move the data to the new CSV file.