The lines 6a and 6b are on the actual 1040 tax return, not when you enter the SSA-1099. You only enter box 5 and any Medicare deductions and federal withholding in box 6 from the SSA-1099.
Enter a SSA-1099, SSA-1099-SM or RRB-1099 under
Federal Taxes (Personal for Home&Business)
Wages and Income
Then scroll down to Retirement Plans and Social Security
Then the second line - Social Security (SSA-1099. RRB-1099) - click the Start or Revisit button
Up to 85% of Social Security becomes taxable when all your other income plus 1/2 your social security, reaches:
Married Filing Jointly: $32,000
Single or head of household: $25,000
Married Filing Separately: 0