Hi Arijan,
The `secrets-update` command is not part of Git for Windows, and never has
been. You must have gotten it from somewhere else.
I _suspect_ that you copied it into `C:\Program Files\Git\mingw64\bin`,
but that directory is "owned" by Git for Windows. In other words, it is
not the correct place to put such a command. Instead, put it into
`%USERPROFILE\bin`.
Ciao,
Johannes