I am an American retiree with some investment and Social Security income. Getting error - "Form 6151: Simplified FTC election must be entered". What's going on?

Yes, this is a foreign tax credit election for the Alternative Minimum Tax, Form 6251.

If you are not in an AMT situation, answering the Form 6251 is largely procedural and will not impact your return. It is coming from the Foreign Tax Paid probably from investments. 

Click the link form more details: What is the Alternative Minimum Tax (AMT) in 2019?

Click this link for an explanation from TurboTax Employee Tax Expert LinaJ2020 about when you should elect to use the simplified AMT Foreign Tax Credit Limitation.

To enter Foreign Tax Paid in TurboTax, it must be entered in two steps.

First, report it on the form as received.

Second, utilize the Jump to link described here: Where do I enter the foreign tax credit (Form 1116) or deduction for 2019?

For a deeper dive into the rules, see page 12 of 2019 Form 6251 Alternative Minimum Tax--Individuals

@mslphd