See Help > About Opera to determine the location of your profile folder, though it is generally along the lines of C:\Users\username\AppData\Roaming\Opera Software\Opera Stable\ where username is your username on Windows - and of course change to Opera GX for the latter's profile.
Wait, you're suggesting that there should be an option such as 'Remind me what my passphrase is'?
If so, then it's absolutely impossible from the technological point of view - it's not anyone bad will, it's cryptography (pure maths actually). These passphrases aren't actually stored anywhere - they are used to encrypt your data before sending them to Sync servers, and Sync itself has no idea what that passphrase could be.
Think of a safe unlocked by a numbers combination - there's no way to know what that combination is just from standing before the safe, unless you completely break it and see what is inside - and this, in case of modern digital encryption, would take a few dozen centuries at least.
@LuisAra Moderators help if possible. And this is a forum for users to (try to) help each other, so although people from Opera read it and reply eventually, you should not expect to get reply, technical or not, from them.
@parsley If you mean the passphrase, first you need to reset your synced data at sync.opera.com.
After that, you go to sync settings in Opera and create a new one.
If it's the regular password, just click on the forgot password link on the login page.