appvsubsystems64.dll
This file is not a system component: microsoft office click-to-run. Restore it from the source that installed it rather than downloading a replacement.
Do not download this DLL on its own
Searching for a missing DLL by name leads to sites that hand you that single file. It is one of the oldest and most effective malware delivery methods there is, and a system DLL from an unknown source is indistinguishable from a deliberately modified one. There is never a legitimate reason to obtain a runtime component that way.
What it is
Part of the Application Virtualization layer that Microsoft Office uses in its click-to-run installations. It is not a general Windows component.
Where you see it: Word, Excel or Outlook failing to start after an interrupted Office update.
Repair Office rather than touching the file: Settings, Apps, Microsoft 365, Modify, then Online Repair. That rebuilds the virtualisation layer properly, which copying a DLL cannot do.
Why there is nothing to download here
There is no verified download we can offer for this one, because installing a file is not the fix. The steps above are what actually resolves it. That distinction matters: the sites ranking for this file name are selling a download that would not have helped even if it were safe.