pdf /a

Hi can you confirm some details?

Can you share the exact version of the software and operating system you are using?

If I understand well, you already have a pdf, do you have access to the original? You need to open the original (MS Word, Adobe InDesign, Quark Xpress....) and export the file with the correct profile.

Can you check if this works for you? PDF to PDF/A — How to convert from PDF to PDF/A | Adobe Acrobat