Yes. A book does not need to be part of a series to benefit from an ebook promotion.
A standalone promotion can help introduce your work to new readers, increase visibility, generate downloads or sales, and potentially lead readers to explore other books you have written.
Series books have an obvious read-through opportunity, but standalone books can still be strong promotional candidates, especially when they have an appealing cover, compelling description, competitive price, and a clear audience.
The right decision depends more on the quality and marketability of the individual book than on whether it belongs to a series.