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Ledger seed phrases are always 24-word phrase (never 12-word), so the seed you are using was not created by a ledger, therefore it is likely unsafe, you are aware of that, right?
If you are using a 12-word seed that was created by a software wallet (e.g. MetaMask), you are taking big risks, just want to make sure you know that this defeats the security of the ledger device.
Or maybe you just got confused, and you restored in your ledger a 12-word seed of another wallet (maybe a Metamask seed?).
In that case, you will never be able to access your ledger accounts that were created with the 24-word ledger seed. In order to get access to your ledger accounts, you need to use your original ledger seed, which (if it was generated by your ledger device) is 24-word long.