This is an error in your schedule C for self-employment/business and more specifically in the income section of your business. If there was an import of information to your business, there may be a stray mark or entry there.
In TurboTax online if the button for "Rebates and Discounts" is selected but no entries are made that may trigger this error as well.
You should "Edit" your business and the "income" section to find the entry that may be causing this error.
You are asked to account for any returns or allowances (or rebates/discounts in TurboTax online). The number needs to be entered as a positive or left blank if it does not apply.