Line 18 will determine if you would like to have more time to pay your self-employment tax ( Eligibility is based on whether you paid household employees or had self employment income, when you earned that income, and whether you owe taxes this year.) If you enter a number there you will have time to pay self-employment tax on a later date. Therefore enter 0 in line 18 and you will be opting out of this option. 

On another note if you did not earn self-employment income or pay household employees in 2020, you are not eligible for this deferral.