This link opens a note in the app.
You are seeing this page because the link opened in a browser instead of in Pistachio Notes. That happens when the app is not installed on this device, or when the link was opened from somewhere the operating system does not hand off, such as a terminal or a preview pane.
The note is encrypted on the device that wrote it
This server stores ciphertext and the keys never reach it, so this page cannot show you the note even if you are the person it belongs to. Opening it needs the app, on a device your account has enrolled.
If you have the app already
Open Pistachio Notes and use the same link there, or find the note by its name. A link opens the note only in a vault you already hold: it names which note to show, and grants nothing on its own.
If somebody shared this with you
Being able to open a note means somebody added your account to the vault it lives in, which is a separate thing they have to do deliberately. If you cannot open it, ask them to share the vault with you.