I had the FP+ error also. I can't pick for myself nor mother, but dad and son can pick.
I cannot choose from the main page, but I found another way:
When at the log-in screen, bring your mouse to hover over (but do not click) the "my Disney experience" and a small window will open, you will see to the left is "Learn About" you will then click on the FP+
You will be brought to a new page for FP+, scroll down to "Get Started Now" and click on the highlighted link for "FP selections" Your dates should show up.
I hope it works!