If you wrote down your 12-word backup phrase but cannot get it to work when you attempt to import it into a wallet app, you may have written down one or more of the words on your list incorrectly.
Usually there is only a small discrepancy between the word you have written down and the actual word of your backup phrase. To determine the correct word match, you may just need to match it to the most similar word actually used in bitcoin wallet backup phrases.
For a full list of the words used in bitcoin wallet seed phrases, you can view the following list: https://github.com/bitcoin/bips/blob/master/bip-0039/english.txt
For other languages, view: https://github.com/bitcoin/bips/blob/master/bip-0039/bip-0039-wordlists.md