Looking at the information shared above, if I understand correctly you are unable to convert a pdf file to excel format? And while converting the file you are getting an error message stating the the pdf file is a secure pdf?
If this is the case then, as per the error message given it seems that its a secure pdf which is restricted to make any changes to it.
You may check- Open PDF>Right Click "Document Properties"> Security> Page Extraction or Document Assembly "Allowed or Not Allowed"?
If Document Assembly or Page Extraction is restricted, then you would not be able to export this pdf. These settings are applied at the time of creating pdf. The security settings of a pdf can only be changed in you own this pdf or have rights to make changes.
Related discussion- Unlocking SECURED files for Editing in Adobe Acrobat Reader DC Mac