Great!! @ilyaM — following your steps fixed the issue! 
Thank you so much for your help 
Steps followed:
- Logged into the Passbolt web UI as an administrator
- Navigated to Admin → Organization Settings
- Located the Metadata Key / Integration Settings / OpenID Metadata section (name may vary by version)
- Clicked “Generate Metadata Private Key”
- Followed the on-screen instructions
This successfully generated and stored the key in the metadata_private_keys table, and the error is now resolved. 