How can somebody be "in his father?"
Jesus was having a good time in the temple, amazing the rabbi's and teachers with his advanced understanding of the Scriptures. He would ask questions that seemed beyond that which would be expected for a twelve-year-old. And when the learned professors would ask him a question they were even more astonished by his answers.
In the meantime, his parents were on their way back home, blithely unaware that Jesus was not among the caravan on its way to Nazareth. At the end of a full day's journey, his parents began searching for him among their relatives and acquaintances, but he was nowhere to be found. The next morning, they had to return to Jerusalem to scour the city for him.
They finally found him in the temple, of all places, dazzling teachers and rabbi's. By this time his mother Mary was perturbed, and said to him, "Son, why have you treated us this way? Look, your father and I have been anxiously searching for you!" (Luke 2:48)
Then Jesus replied in a peculiar way. He responded,
"Why were you looking for me? Didn't you know I had to be in my Father?"
"In my Father?" What does that mean?
Many translators struggled with that, unwilling to leave Jesus' answer as found in the original Greek. They attempted to 'fix' Jesus' answer by adding more words so that it made more sense to them.
Some translators rendered it "Didn't you know I had to be in my Father's house?"
Or worse, others put it "Didn't you know I hade to be about my Father's business?"
(The better translations put these added words in italics to mark them as not present in the original.)
The better course would have been use of the maxim, "Let Scripture define Scripture." It's a short hop to the book of John, where we find Jesus telling us what "in my Father" means, at John14:20.
"In that day you will know that I am in my Father, and you in me, and I in you"
This bears closer examination. Here is John 14:20-21a, saying "In that day you will know that I am in my Father, and you in me, and I in you. Whoever has my commandments and keeps them, he it is who loves me. And he who loves me will be loved by my Father, and I will love him and show myself to him."
When Jesus is in the will of the Father, he is in his Father. When you and I keep Jesus' commands, we are in Jesus. When we are in Jesus, we are loving him. And we who loves Jesus will love the Father, and Jesus will love and reveal himself to us.
So what went wrong?
Jesus knew where he had to be, but this twelve-year-old jumped the gun. There is a time for everything, and the time for Jesus to begin his ministry had not yet begun. Instead, it was time for Jesus to finish growing up, and gain more wisdom (Luke 2:52).