Use of unassigned variable?

Attempting to get a constant spin on a game object. im pretty new, what am i doing wrong?

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.SceneManagement;

    public class BannerHoldersScript : MonoBehaviour {
    public GameObject BebopPropeller;
    void Start() {
        float propellerRotate = 0;
        float propellerVelocity;{
        if (Input.GetKey(KeyCode.UpArrow))
        propellerRotate = Mathf.SmoothDamp(propellerRotate, 1440, ref propellerVelocity, 0.8f);