Is the verb ‘miss out’ separable or inseparable?
e.g. I missed it out. or I missed out it.(the party)
“I missed it out” is correct; “I missed out it” is wrong.*
However, it seems unusual to say “I missed it out” when referring to a party. You would normally say “I missed it”, or perhaps “I missed out on it” (the latter has a stronger sense that you were deprived of an enjoyable experience).
*But "I missed out the " is OK.