The error lies in the verb construction. In English, the modal verb "will" must be followed by the base form of the verb (without "-ing") to indicate the simple future, or it must be part of a continuous structure.
The Error: "Will singing" is grammatically incorrect because a modal verb cannot be directly followed by a present participle (-ing form).
The Correction: There are two ways to fix this sentence depending on the intended meaning:
Simple Future: "Sarah will sing at the concert..." (Plan/Fact)
Future Continuous: "Sarah will be singing at the concert..." (Action in progress)