Acrobat Standard DC will no longer password protect documents

[admin-law]

...without paying for it.  It has worked on this computer for a long time, now it will not.

What has changed?  Why am I being prompted to pay for something that has been working at no charge?

[admin-law]

More information..... If I open a PDF from Explorer, I cannot password protect.  If I open Acrobat DC, do a file...open... then select a PDF, I can password protect....

Confused.

[Test Screen Name]

Compare the app which is running. Look under Help > About. My money is on you having a copy of Acrobat Reader (free) installed along with Acrobat Standard (definitely not free).