What is checksum used in BIP-39 and what is it for? I don’t trust programs that generate 12 words, since I don’t know what algorithm they are working on, and for this I wanted to take 12 words from the total 2048 bip-39 word list, but if I do, and after that I want to get using my words to access my wallets, for example using this script https://github.com/iancoleman/bip39 I get an error Invalid mnemonic.

I'm in a dead end, I'm afraid if I use the 12 words that I chose from the general list, because of the checksum error, I will lose my bitcoins in the future, please clarify. I'm at a dead end, I'm afraid if I use the 12 words that I chose myself from the general list of words, I will lose my bitcoins in the future because the programs will not want to accept my words, because of the checksum error. You can help me? How can I convert my 12 words that I chose myself, from a list of 2048 words, into a wallet?

At the moment I can choose any 12 words at my discretion, and Electrum will accept them, but with an unkown wordlist error.

Can you explain to me how the checksum check works, and what is it for in BIP-39?