If you check the BIP39 and the 'next' still grey out after entering your seed phrase, then that seed phrase is not correct.
If you check "BIP39" in the desktop version of electrum, it's impossible that the "Next" button is grayed out.
If you enter a valid BIP39 seed phrase (I mean a BIP39 seed phrase passing checksum), electrum shows a message saying "BIP39 (checksum: OK)"
If you enter an invalid seed phrase, but all the words are still in BIP39 wordlist, electrum shows a message saying "BIP39 (checksum: failed)"
If you enter words that are not in BIP39 wordlist, electrum shows a message saying "BIP39 (unknown wordlist)"
In all three cases, electrum allows you to create a wallet.
If you check "BIP39" in the mobile version, electrum allows you to generate a wallet only with a valid BIP39 seed phrase.
To OP:
If you check BIP39 in the desktop version and electrum displays a message saying checksum is failed, it still allows you to click on "Next" button and create a wallet, but that doesn't mean you have entered a correct seed phrase.