Hello @Bruce K
The PIN prompt you’re seeing is the Windows sign‑in PIN, not an Office‑specific PIN.
It is the PIN that you use to Login to the Device.
what’s happening is that Windows is enforcing its Hello sign‑in method (PIN, fingerprint, or security key) before letting apps like Word/Excel use your Microsoft account.
- Go to Settings → Accounts → Sign‑in options.
- Under PIN (Windows Hello), see if a PIN is set.
If its set you need to enter this PIN.