If you have a Brave Wallet 24-word seed phrase, you can only use it in Brave’s wallet.
Unfortunately, Brave’s 24 words seed phrases are non-standard (not BIP39 compliant), meaning Brave’s seeds are incompatible with MetaMask, MyCrypto, MEW, and other wallets. If you want to switch to a MetaMask wallet you must send your funds to a brand new MetaMask wallet address.
Make sure you backup the new seed phrase and store it in a safe place.
For more information regarding this, please visit this issue posted on Brave’s Github Repo here.