If you don't choose a job for your tamagotchi quickly enough, you should wait for another job notice. The job notice will come back in a couple of hours or in a couple of days.
Not to worry...there will be another job offer. Even if you did take that first job offer that you missed, your Tama is going to have to "quit" that job and move on to a different one in a few days. A Tama has more than one job throughout it's lifespan.