What is the ruling on one who recites Al-Fatiha but does not extend "wa la ad-dalleen"?

Cursing whoever does not extend the word (الضالين - Ad-Dalleen) has no basis. Shortening the vowel in it is a subtle error (lahn khafi) that should be avoided, but it does not necessitate cursing or expulsion from Allah's mercy. It is not an individual obligation (fard 'ayn) that entails punishment. Therefore, your prayer is valid, and there is no sin upon you. It is advised to be diligent in reflection (tadabbur) and attentive to the rules of Tajweed.