If you enter the 12th word randomly, it's very very unlikely that the seed phrase is valid. As stated by bitmover above, even if the seed phrase is BIP39, it must pass the checksum.
For a 12 word BIP39 seed phrase, it's actually not that unlikely. Given that for a 12 word phrase the checksum is 4 bits long, then you have a 1 in 2
4 chance, which is 1 in 16. With 2048 words in the BIP39 wordlist, that means that 128 of them (on average) will produce a valid seed phrase.
For a 24 word BIP39 seed phrase the checksum is 8 bits, so the chance becomes 1 in 2
8, which is 1 in 256. For an Electrum seed, the chance of finding the right version is 1 in 2
12, or 1 in 4096, as I've explained above.
If you want to force electrum to generate a wallet with any words, click "options" and check "BIP39 seed".
If you don't want to use the BIP39 wordlist, then a better option is to import your own wordlist in to Electrum (replace the file english.txt at electrum/electrum/wordlist with your own wordlist), and then load Electrum normally and have it generate a new seed phrase. Your new custom seed phrase will be importable in to any Electrum client, even if they do not know the wordlist you used.