The error message "Data source reference is not valid" usually appears when Excel cannot find or access the data source that you are trying to use for your pivot table. This can happen for a number of reasons, such as:

1. The data source has been moved or deleted: If you have moved or deleted the file or worksheet that contains the data you want to use for your pivot table, Excel will not be able to find it.
   
2. The data source is in a different format: If the data source you are trying to use is in a different format than what Excel can recognize, such as a PDF or a Word document, you will get this error message.
   
3. The data source is password protected: If the data source you are trying to use is password protected, Excel will not be able to access it without the correct password.
   
4. The data source is corrupted: If the data source you are trying to use is corrupted, Excel will not be able to read it and you will get this error message.

To fix this error, you can try the following steps:

1. Check that the data source is still in the same location and has not been moved or deleted.
   
2. Check that the data source is in a format that Excel can recognize, such as a CSV or Excel file.
   
3. Check that the data source is not password protected or that you have the correct password.
   
4. Try opening the data source separately to see if it is corrupted. If it is, you may need to restore a backup or recreate the data source.

Once you have resolved the issue with the data source, you should be able to create your pivot table without any further issues.