What you're trying to do makes no sense.
The first problem is that story points have no relation to time. Story points are a measure of effort needed or complexity. Something that requires a lot of effort may be something that can also be done very quickly. However, something that is estimated to be a very small amount of effort may be done with the same amount of working time or much longer. You simply can't compare an estimate of effort or complexity to time.
The second is that Scrum doesn't have the concept of a project schedule. In Scrum, each Sprint is a timeboxed activity that could be viewed as a project. At the end of the timebox, work is either done or not done. You deliver at least one potentially releasable Increment every Sprint. The only variance is in the scope of what is delivered and rarely, if ever, in the minimum frequency of delivery.
I would recommend assessing why you wish to use schedule variance. I do not believe that it is appropriate for situations that Scrum, and other Agile methods, are appropriate. Agile methods are best used when there is a good deal of uncertainty. With uncertainty, it's not effective to make long term plans that you would use to calculate variances against.