Problem

Your printer is visible in network share (network computer, NAS…), but you are unable to install the printer due to the following error:

Windows cannot connect Print processor does not exist

This subsequently causes issues with document conversion.

Environment

Windows 10 (2018 October update)
Synology NAS
an old USB-only printer connected to Synology NAS with printer sharing enabled
basic driver downloaded from manufacturer website and unzipped to folder (.inf and .cab files)

The printer was HP DesignJet T610 44in, currently unsupported by HP. The inf file says: „HP Designjet Z110x series driver INF file for Windows XP and Windows Server 2003 64-Bit Edition“.

Solution

Basically, you have to use a different way to add your printer to the system.