^ Read it up how to write message (sign message), you should provide your name, time and etc.

Quoted and verified

That's the default address and private key pair when you generate a brainwallet without any texts.
Try it: https://brainwalletx.github.io/#generator

And looks like he just copied that signed message from a tutorial somewhere.

i save the private key

Create another address. Your staked message will be meaningless because of the reason nc50lc mentioned above.

To put it simply, anyone can access your address because the private key is public already.

Don't use brainwallet, it's an online service and any private key you generate using that service could be compromised. I've found the easiest way to create a secure wallet is by using Electrum, but you must still be careful as there are a lot of fake versions.  Only download the software from electrum.org, and learn to verify the download.  There's a lot of information in the electrum sub-board, here: https://bitcointalk.org/index.php?board=98.0

Lost that adress. Now im using blockchain wallet which one i cant sign

You can sign with it, but not using the default HD blockchain.com wallet. It requires a bit of a workaround and you would need to export your private keys out of blockchain wallet and then import them into another wallet and use that... for instance:

1. Use your 12 word backup phrase from blockchain.com and create a "BIP39" wallet within Electrum. Electrum would then have the same addresses as your blockchain.com wallet and would let you sign using your "blockchain.com" addresses

or

2. Use your 12 word backup phrase from blockchain.com with an (offline) copy of Ian Coleman's Mnemonic code converter. That will give you the private keys for your blockchain.com addresses, you can then use those private keys and import them back into blockchain.com... it should let you sign a message with one of those imported addresses.

NEITHER of these options should be used lightly and without taking adequate precautions to secure your 12 word backup phrase and/or private keys.

Quoted, and verified with Electrum wallet.

Quoted... but it doesn't verify in Electrum. Please double check your message (make sure there are not trailing whitespaces etc) and the signature used.

EDIT: Figured it out... there is a trailing space after the date... so it should be "... Today is April 04, 2020. ". Then it verifies.