Solution: You'll get invalid header errors mostly when there is a mismatch between the type how you call the Attributes or/and Attribute Group or/and Option Values or/and Attributes Group.
For example: In the bulk excel sheet that you import, if the Attribute Group is called as Name, then in the backend setting(Tools > Import/Export > Settings), this field 'Use attribute_group_idinstead of attribute_group name in worksheet 'ProductAttributes' should be unticked. If you are importing the sheet with Attribute Group ID and NOT by Group Name, then this field should be checked.
Same is the case with other fields where you may get error i.e., Option Values, Option Group or Attribute values.
To simply understand, a student in a class will have two unique identification properties i.e., Name and Roll Number. If a student database in a system is having the header as Roll_Number, then only the Student's Roll Number should be entered and NOT Student Name. Hence this is more like a configuration setting rather than an error.
This happens when there is a no product_id mentioned in the main 'Products' sheet but exists in any other sheets/tabs like ProductAttributes sheet or ProductOptions sheet, for instance.
Solution: Find a Product_ID which exists in any of the sheets, but not in the 'Products' sheets. If you don't need those Products to be imported, either remove those product_id rows from the other sheets or add that product_id details in the 'Products' sheet.
Sometimes MS-Excel adds invisible formatting to the empty rows and or columns.
Solution: Hard-Delete the empty rows and columns and import again. You can do hard-delete by selecting all the empty rows together, and in the left hand side, do a right click and click on delete. This not just delete the content in the empty rows, if any, but delete the entire rows and any invisible formatting applied. Do the same for the empty columns too and upload again.
We understand that sometime it would be annoying importing products in bulk, but it's mostly some common errors we make which you will figure out when you keep working with the system regularly.
In rare cases, when you are not able to import the sheets at all, we recommend you to 'export' some of the existing products and just change the data there without doing any other change. This should work 100%.