Issues with built-in apps in Sequoia 15.5 (Migration Assistant, iCloud, Password Manager)

It does not sound like the Mac was properly reset before being sold to you or reset by you to set it up as a new device.

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

iCloud requesting access to a Mac which is not one of yours is concerning. That means that there is encrypted data in iCloud for that account from that Mac it is asking for and it will not sync to the current Mac until that has been authenticated. Are you using an Apple Account that was created by you, or was there already one signed into the Mac?

Ok, that looks like the Mac is wanting to sync encrypted content that was created with your Apple Account signed into another Mac. That Mac may no longer be in your Device list, but the data that synced to iCloud is still stored in iCloud. Passwords are end to end encrypted, so those stored in iCloud from your Apple Account will not sync back to your new Mac until you have provided the Login Password for that Mac.

On your Mac, go to  > System Settings > [name on top], and click See All. Does it show Passwords & Keychain as being turned on? If not, that explains why you are unable to sync new Passwords when you tap on it, you should see the option to turn it on. If you are unable to provide the Login Password to continue and sync the content, then you should see an option to delete the encrypted data in iCloud, so you new passwords will be synced. If that option is not given right away to delete the encrypted data, then make a guess and the option may appear after it has failed.

Could you post the screenshot of the request you are seeing, that may be helpful to determine the reason why it is asking for it. Please redact any personal information including the serial number of the device being requested.