Learn Spanish (Spain): No encuentro mi carné.

In Spanish, the simple present tense is often used to express that you are failing to do something right now. While you could say No puedo encontrar (I cannot find), No encuentro (literally "I do not find") is much more natural and common in everyday spoken Spanish.

Why is there a "ue" in encuentro if the dictionary verb is encontrar?

Encontrar (to find) is a stem-changing verb. When you conjugate it for most subjects (like I, you, he, she, or they), the o in the root of the word changes to ue. That is why "I find" is encuentro instead of "encontro".

Does carné specifically mean an ID card?

Yes, in Spain, carné (sometimes spelled carnet) refers to an official document in card format. It usually means an ID card, a student card, or a driver's license (carné de conducir). It is a borrowed word from French, which is why it has an accent mark on the last letter to show you must emphasize the final e.

Do I need to put Yo (I) at the beginning of this sentence?

No, it is not necessary. Because encuentro is specifically the "I" form of the verb, the subject is already perfectly clear. You would only add Yo at the beginning if you wanted to strongly emphasize that you are the one who can't find it.