Even if this works fine in JS, as Stylesheet.create simply returns the object passed as a parameter, it throws a typing error in TS. The method is typed as followed:

export namespace StyleSheet {
  type NamedStyles<T> = {[P in keyof T]: ViewStyle | TextStyle | ImageStyle};

  /**
   * Creates a StyleSheet style reference from the given object.
   */
  export function create<T extends NamedStyles<T> | NamedStyles<any>>(
    styles: T | NamedStyles<T>,
  ): T;
}

Here you can see that it takes an object as a parameter where each value must be of type ViewStyle | TextStyle | ImageStyle.
As mentioned by @DanielGabor you can still create a style object without using Stylesheet.create where keys return functions that themselves return ViewStyle | TextStyle | ImageStyle