My 12-word seed doesn't work in Bitcoin Core
You entered your 12-word seed into Bitcoin Core, your balance doesn't show up, and you think you did something wrong or lost your bitcoin.
Bitcoin Core never supported BIP-39 phrases: you can't restore your 12 words there natively.
Why Bitcoin Core won't let you restore the seed
Bitcoin Core, the reference software for Bitcoin, stores its backup in a wallet.dat file or descriptors, not in a 12-word phrase. It never implemented the BIP-39 standard, so there's no field to paste your 12 words and derive your keys.
So if you open Bitcoin Core expecting to recover your wallet with the phrase, you'll find that option doesn't exist. It's not that you're using the program wrong: the backup format is different.
So where are your bitcoin?
Your bitcoin don't live inside Bitcoin Core or any wallet: they're on the blockchain, tied to the keys derived from your seed. If your seed is correct, you can regain access from another wallet that supports BIP-39.
You don't need Bitcoin Core to accept your seed to get your bitcoin back. You just need a BIP-39 compatible wallet, like the one you probably used to create that seed.
How to access your bitcoin without Bitcoin Core
Install a wallet that supports BIP-39 and use the restore or import 12-word seed option. After entering it, you'll see your transactions and can move your bitcoin.
The source for this fact is: Repositorio de Bitcoin Core (pedidos de BIP-39 cerrados) + docs de wallets de Core.
What this doesn't mean about your 12-word seed
Bitcoin Core can't read your seed because it uses a different backup system, not because your seed is wrong. Almost all modern wallets do support BIP-39, so those 12 words are still your key to recovering bitcoin in a compatible wallet.
Source: Repositorio de Bitcoin Core (pedidos de BIP-39 cerrados) + docs de wallets de Core