The IRS made major changes to Form 1040 for 2018, so TurboTax had to change what tax situations are included in Free Edition. Some of the things that used to be included are no longer included.
The IRS has simplified Form 1040 by moving many of the entries that used to be on Form 1040 to a set of six separate schedules. There are no short forms 1040A or 1040EZ for 2018. All 2018 tax returns will be filed on Form 1040. TurboTax Free Edition for 2018 will only prepare simple tax returns that can be completed with Form 1040 itself. Anything that requires one of the six new schedules requires an upgrade.
So Schedule D is not the only thing that would require you to upgrade. You must have entered something that requires one of the six new schedules. There are dozens of things that it could be. If it told you which schedule you need, I can give you a list of what's in that schedule.
