If you are a US citizen then you are required to report your total worldwide taxable income even if there are no tax reporting forms issued.
If you are new to being self employed, are not incorporated or in a partnership and are acting as your own bookkeeper and tax preparer you need to get educated ....
If you have net self employment income of $400 or more you have to file a schedule C in your personal 1040 return for self employment business income. You may get a 1099-NEC for some of your income but you need to report all your income. So you need to keep your own good records. 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
Publication 463 Travel, Gift, and Car Expenses
https://www.irs.gov/pub/irs-pdf/p463.pdf
Publication 946 … Depreciation
https://www.irs.gov/pub/irs-pdf/p946.pdf
You are self-employed in the United States. The income that you get from freelancing is U.S. income, not foreign income. The fact that your client is in another country doesn't matter.
The tag below your question indicates that you are using TurboTax Premier Online. In Premier you cannot enter self-employment income without a 1099-NEC, and you cannot enter any business expenses. You have to either upgrade to TurboTax Self-Employed or use any version of the CD/Download TurboTax software. If you use the CD/Download software, you should use TurboTax Home & Business (not TurboTax Business, which is a different product), because Home & Business will give you more detailed guidance and help for reporting self-employment.
Does your German client pay you in U.S. dollars? All amounts that you enter in your tax return have to be in U.S. dollars. If you receive payment in another currency you have to convert the amounts to dollars.