Yes it's perfectly fine to be sad. You are a human with a range of emotions that are in you to help you survive and thrive. Don't stay in sadness for long as it can be unhealthy. But definitely don't ignore it or think it's gonna sabotage your LOA efforts.

If the universe (which is not a sentient creative being like God) is so fickle and sensitive to our human emotion, then it needs to go f itself. God knows the heart and desires of every single person, if you don't get what you desire there's a good chance it wasn't good for you to begin with.

Hindsight is always 20/20