I checked the box 7 for multiple codes (7D) but no way to enter the 2nd code appears.

In TurboTax Online the entry for the Form 1099-R, there is a checkbox for more than entry in box 7. If you check the box, it should open another entry field.

If you are using Turbo Tax Online, the most likely reason for buttons that don't work or not being able to enter information is that something in the web browser is not displaying the page properly.  The easiest way around this is to simply use a different web browser.  You can select from Google Chrome, Mozilla Firefox, Internet Explorer or Safari.  If you need to install another browser, be sure to go to the trusted home site for the software for  security reasons.

If you are using the Turbo Tax Phone App, then try opening Turbo Tax in the browser window on your phone. It will use the same login information but may display the page better.