Final answer:
The correct phrase is "My birthday outfit isn’t going to be here in time," as it means the outfit won’t arrive early enough for the intended occasion.
Step-by-step explanation:
The correct grammar for expressing something happening sufficiently early to avoid being late is "My birthday outfit isn’t going to be here in time." Therefore, the correct preposition to use in this context is in, not on.
The phrase ‘in time’ typically means ‘early enough’, whereas ‘on time’ suggests something happens at the scheduled or expected time. In the context of your birthday outfit arriving, if it's not arriving ‘in time’ for your birthday, it means it will not arrive early enough for you to use it for the occasion you intended.