There are two possibilities in this situation:
The Skill Is Desired, But Not Required, For The Job
Sometimes a company will list a skill under a job listing, with the understanding that they would like to have a candidate with that skill, but it is not a requirement. This may be a skill that is needed only irregularly that you could be trained in, or a skill that is tangentially related to the work you will be doing.
In this case, you will want to sell yourself on your ability to pick up this skill quickly, and apply the part of it needed for your job. If you have skills that are adjacent to it, that helps to convince them that you can learn it easily.
Which brings me to the second possibility...
They Believe You Have Skills Equivalent To That Skill
It's possible that because you listed a different skill that is related to that skill, that the company believes you are able to perform that task as well.
Only you can determine if this is true - if you think you are capable of performing the task that they have outlined regularly, you can go in and tell them that, because of your related skill, that yes you can do it - or that with some training you could do it as an expert.
If you believe you are not capable of that skill, or not able to be trained in it properly - this is a judgement call I would not make lightly. If the company is willing to train you in that skill, it is beneficial to you to take that training.
But if you truly believe that skill is outside of your ability to learn on a fundamental basic level, you should consider letting the company know that, politely. It's possible they did not realize it was outside of your wheelhouse.
Again though, explore the possibility that it could be an adjacent skill to something you do know first. They picked your resume, so they clearly saw something in there that they believe shows you have the talent they want.