We apologize for the delay in response to your query.
As the error message shared above, it seems that you are experiencing this problem with a specific file? Have you checked with a different file? If not, then you may try converting a different pdf to excel.
The conversion resulting this error, there might be couple of possibilities behind it-
1- The pdf file is a secured pdf for which any changes are restricted. You may open the pdf Right Click>Document Properties>Security>Page Extraction and Document Assembly and checkwhether its allowed or not? If its restricted then you would not be able to convert the pdf or make any changes to it.
2- The pdf security can only be changed if you have the permission to change it or by the owner.