You have 11 words and you want to select the 12th word, so the BIP39 seed phrase pass the chacksum. Am I right?

If I have understood you correctly, first of all note that that's not how a BIP39 seed phrase is generated.
It's not that 11 words are generated and then the 12th word is selected. Instead you generate a random number and your seed phrase represents that number.

Anyway, if you have the first 11 words and you want to have valid BIP39 seed phrase, there are 8 words that can be used as the 12th word.
To find that word, you should use brute-force method. This means that you should test all the 2048 words one by one.