Read line 18 again. That's only if you have an amount on line 19. Otherwise you skip line 19 and go directly to line 20.
Line 18 says.........
Taxable social security benefits. Enter the smaller of line 16 or line 17 18
If prior year lump-sum benefits were received, go to line 19, otherwise,
skip line 19 and enter the amount from line 18 on line 20.