If you're trying to restore your account on Polkadot-JS UI or the Polkadot extension and you're getting an error like this:

Polkadot-JS UI

Polkadot extension

make sure that you check the following:

1. All the words are spelled correctly. You can check them against this list to ensure they are spelled properly.
2. You enter them in small letters only
3. You use only one space between them and no space or new line at the end
4. Your keyboard is set to English
5. That all the words are entered. Until you fill out the whole mnemonic phrase correctly the error message will show up.