Never misrepresent your skills in a job interview. If you are asked about your skill level with something, it is highly likely that you will be expected to demonstrate that skill on the job. If you misrepresent your ability, you will be found out and it will go badly for you.
Saying that you don't have a skill they ask about is not always fatal to your application. Sometimes the skill is not required but is a plus. Even if the job listing says it's required, you might still end up being the best applicant, and they may decide they're willing to give you time to learn that skill.
You could ask, "Is this skill required in order to be accepted for this position, or is it something that I could learn after I'm hired?" If they say that you could still be hired, make sure to point out instances from your work history where you learned new skills for the job. Also mention any related experience and skills you do have.
If the employer will not train you in the skill and is unwilling to give you time to learn it, or if you are not able or willing to learn that skill, it is in everyone's best interest to politely terminate the interview process, at least for that position. You could say something like, "It seems this position isn't a good match for my skill set. Is there a different position that would be a better fit?"