your capital gains (assuming they're all long-term) are not being taxed it's your earned income
total income $36,000
less; capital gains $18,000
standard deduction single $12,550
income on which tax computed $5,450
tax approx 10% $545
if this is about the tax you see Turbotax is correct.
otherwise, you would likely be seeing a tax of over $2,600
