How can you get a mnemonic phrase wallet key (12 words login key) from a private key? actually I Will create a mobile app in flutter and a web app in react js so can you please guide me
It's the other way around. Multiple private keys can be derived from the mnemonic phrase, but it's not mathematically possible to derive the phrase from a private key.
See the original BIP39 standard introducing the mnemonic phrase: https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki