For each PDF/A-1 validation issue we try to explain what it means and, if possible how to resolve it. For a more technical explanation of the issue we quote the related section of the ISO 19005-1 PDF/A Specification.
The most common cause of this failure is attempting to password protect a PDF/A file. Password protected or encrypted PDF files are not PDF/A compliant.
How can I fix it?
Assuming you know the password, you can use Solid PDF Tools to remove the protection from your PDF file and then convert it to a validated PDF/A file.
Document | Security - to remove protection
File | Save As PDF/A - create a validated PDF/A file
6.1.3 - ISO 19005-1 PDF/A Specification:
The file trailer dictionary shall contain the ID keyword. The keyword Encrypt shall not be used in the trailer dictionary. No data shall follow the last end-of-file marker except a single optional end-of-line marker. The file trailer referred to is either the last trailer dictionary in a PDF file, as described in PDF Reference 3.4.4 and 3.4.5, or the first page trailer in a linearized PDF file, as described in PDF Reference F.2. In a linearized file the ID keyword shall be present in both the first page trailer and the last trailer dictionaries and the value of both instances of the keyword shall be identical.
NOTE: The explicit prohibition of the Encrypt keyword has the implicit effect of disallowing encryption and password protected access permissions.