ALL  your gross  sales/income from all sources are entered on the Sch C as income ... then you will deduct expenses and inventory costs to come to a net profit on the Sch C .... seems you entered the net income on the Sch C and the IRS could not relate the 1099-S income to the Sch C.  If the IRS does an income audit on your business then they will catch any income you do not report even if you don't get a 1099 for it ... so always report all your income. 

This will require an amended return in order to address your error and the IRS changes.  You REALLY should seek local professional assistance in this matter.   If you use the Turbotax LIVE option that is only operational when the Online version is operational  and the online program has shut down for the conversion to the 2019 program.   

Contact TT support if you have questions on the LIVE option:   

See this TurboTax support FAQ for a contact link and hours of operation -https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number

If you already have a CPA then they should tell you what/where/why/how to fill in the Sch C ... 

How do I report income from self-employment?

If you received a 1099-MISC for your self-employed income

Type 1099-misc in the Search box and then select the Jump to link. This will take you directly to the section where you can enter your 1099-MISC. You’ll also be able to enter any cash, personal checks, credit card payments, or cryptocurrency (Form 1099-K) related to your self-employment.

If your self-employment income was only from cash, personal checks, credit card payments, or cryptocurrency 

Type Schedule C in the Search box and then select the Jump to link. This will take you to where you can enter any cash, personal checks, credit card payments, or cryptocurrency (Form 1099-K) related to your self-employment. You may first be asked some general questions about your business. After you answer them, you’ll be taken to the Let's get income for (your line of work) screen where you can enter this income as Additional Income.

Related Information:

• What's the difference between self-employment income and other income?
• Where do I enter a 1099-MISC?
• Why am I paying self-employment tax?
• How is cryptocurrency like Bitcoin taxed?

What if I didn’t get a 1099-MISC, but made money from self-employment?

If you weren’t paid $600 or more, a business isn’t required to send you a 1099-MISC, but you still need to report the income.

To enter your self-employment income not reported on a 1099-MISC:

1. When in your account, select the Take Me to My Return button on the main screen.
2. Type “Schedule C” in the Search box
3. Select the “Jump to” link.
4. Answer the general questions about your business (or select Edit at the work summary screen).
5. At the Let’s get income for…screen, select Additional Income.

If you did make $600 or more, contact the company or individual you worked for.

Related Information:

• How do I report income from self-employment?

Here is some reading material……

IRS information on Self Employment….
http://www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/Self-Employed-Individuals-Tax-Center 

Publication 334, Tax Guide for Small Business
http://www.irs.gov/pub/irs-pdf/p334.pdf 

Publication 535 Business Expenses
http://www.irs.gov/pub/irs-pdf/p535.pdf 

Home Office Expenses … Business Use of the Home

https://www.irs.gov/businesses/small-businesses-self-employed/home-office-deduction

https://www.irs.gov/pub/irs-pdf/p587.pdf

There is also QuickBooks Self Employment bundle you can check out which includes one Turbo Tax Self Employed return and will help you keep up in your bookkeeping all year along with calculating the estimated payments needed ....
http://quickbooks.intuit.com/self-employed