Hi filippo146,
Thank you for using the Apple Support Communities! I understand you are unable to unlock a note, and I'd like to help.
Do you have your note saved locally to your, or to iCloud? If saved to iCloud, try logging into iCloud from a computer browser, and see if you are able to unlock the note from there without issue: View notes and locked notes on iCloud.com - Apple Support. Let me know the result to this.
Which device model are you using to access this locked note? Try restarting the device: Restart your iPhone - Apple Support, and then see if you are able to unlock your note after that. I also would like you to create a new test note to see if you can lock and unlock it without issue: Lock notes on your iPhone, iPad, iPod touch, and Mac - Apple Support.
Keep me posted on the results. Regards.