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