For making the coroutine wait for a few seconds before executing isCoyote = false, you will need to use new WaitForSeconds().
What this function does is it waits for that delay and then executes isCoyote = false.
Try this code:
private IEnumerator ApplyCoyote()
{
isCoyote = true;
yield return new WaitForSeconds(coyoteDelay);
isCoyote = false;
}