UPDATE: This problem is fixed in Monterey, which restores writing permissions to PDF Services. The solution is thus to update beyond Big Sur.
The problem is that as of Big Sur, PDF Services has been massively sandboxed so as to render it useless.
error 09:33:50.180022+0100 kernel Sandbox: printtool(5086) deny(1) file-write-create /Users/Shared/Print/Out/big sur - Fix-restore "Print to PDF" to preset location in Big Sur - Ask Different.pdf
The only thing I have found that works is an Automator Application (nb: not 'workflow' or anything else). Presumably, applications have local code-signing or something that lets them through the sandbox. So you will need to create an Automator sequence that saves the file to your destination.
The problem also affects executables, like shell and python scripts. Automator Print plug-ins no longer work either.
Relax the sandbox
However, this sandboxing can be 'fixed' by adding the words
Sandboxing Relaxed to
/etc/cups/cups-files.conf. You can do this from within the CUPS web front-end (Administation tab > Edit Configuration File button), or just with Terminal or with BBEdit etc. You'll need to restart cupsd if you don't use the web front-end.
This seems to allow
printtool a bit more freedom, but the executables it calls are still subjected to various restrictions.
Please 'send feedback' to Apple.