You must have entered income that TurboTax is recognizing as self-employment type income, either as cash or from a 1099-NEC, 1099-MISC or 1099-K form.
When your self-employment income is $400 or more, after deducting expenses, Schedule SE will generate in TurboTax.
Please see the TurboTax Help article Why am I paying self-employment tax? for more information.
