You are trying to access data from an encrypted directory. To do this, log in as the user owning the data in question (eg sally for
/home/sally). If the home directory itself is encrypted, logging in will automatically decrypt the data. If it is a Private directory, there should be a launcher in that folder to click (which may again prompt you for the user password).
If the "Access Your Private Data" launcher fails for some reason, use this command and follow instructions:
If, for some reason, you have the data in question independent of the system it was used in, you will need the private key that Ubuntu's encryption wizard asked you to save on installation.
If you have neither the password for user with that encrypted directory, nor the private key, I'm afraid there is no way to recover the data.