If I'm voluntarily withholding 7% tax from my Social Security, why do I owe federal taxes after putting in my Social Security retirement income?

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

Social Security is on 1040 line 5a and the taxable amount on 5b.

To see the Social Security Benefits Calculation Worksheet  in Turbo Tax Online version you would have to save your return with all the worksheets to your computer.  Or if you are using the Desktop program you can switch to Forms Mode (click Forms in the upper right) and click on SS in the list on the left side.