General:
* I am missing something.
* I am lacking something.
* I need something.
* Something is missing.
* I am incomplete.
Specific:
* I am missing you. (If you are talking about a person)
* I am lacking motivation. (If you are talking about a feeling or quality)
* I need more information. (If you are talking about information)
Formal:
* I am deficient in this area.
* I have a shortcoming in this regard.
The best translation will depend on the specific context of your sentence.