Testnet wallets in Blockstream Green operate on a separate network than the Bitcoin blockchain, and are used solely for testing purposes. Testnet wallets allow developers or testers to experiment without having to risk actual bitcoin.
The funds in a Testnet wallet do not have any real value, and the testnet bitcoin in these wallets cannot be "activated" or "converted" into mainnet bitcoin. If you try to send testnet bitcoin to a bitcoin wallet, you will receive the "invalid address" error.