Where can I find the seed phrase/Mnemonic?
The seed phrase is issued when registering a new wallet. If you did not save it, but you have a username and a password, log in to the console and open this page: https://testnet.console.minter.network/account.
In the field "Mnemonic" you can see and save the seed phrase.
In case you forgot both the password and the seed phrase, you will have to create a new wallet.